openzfs: Add support for Solaris style extended attributes

[rmacklem]

Summary

I've put the patch here first. Once this review is completed, I will
take it over to OpenZFS and discuss it there, plus propose a pull request.

Since ZFS was developed for Solaris, Solaris style extended attributes
(which I call named attributes, which is the NFSv4 name) are already
implemented in OpenZFS. This VFS/KAPI interface presents a named
attribute directory that is associated with a file object, which holds
regular files that are the attributes. The directory is read with getdents(2)
or getdirentries(2) and the attributes found in the directory with regular
file I/O such as read(2), write(2), lseek(2) and ftruncate(2).

This patch provides the changes to use this alternate KAPI to access
and modify extended attributes through the FreeBSD VFS, as patched
by commit 2ec2ba7e232d to main/freebsd-current.

A few notes:

• Since the named attribute directory is associated with a file object

found under the same name in the directory tree, name caching must
be disabled.

• Two new flags called LOOKUP_NAMED_ATTR and V_NAMEDATTR are

used to indicate that zfs_zaccess() must check permissions for the
attributes.

• The permission checking code in zfs_zaccess() that is used when the

above flag(s) are set is cribbed directly from zfs_zaccess() in the
Linux branch. (openzfs/module/os/linux/zfs/zfs_acl.c).

• zfs_check_attrname() was not changed. It was simply moved up

in the source file so that it could be called in zfs_freebsd_create().

The new named attribute KAPI is supported when the "xattr" property
is set to "on"/"dir" (I think they are synonymous?). Setting "xattr" to "sa"
or "off" disables it.
If this is not felt to be sufficient, a new setting for the "xattr" property
could be defined.

Note that the extended attributes can be manipulated by either KAPI
(this Solaris-like one or the FreeBSD/Linux extended attribute model).
The only limitation is the size restriction enforced for extattr_get_[fd|file]()
and extattr_set_[fd|file](). Storage of extended attributes does not change
and this works fine for the "dir" version (but not the "sa" version, as noted above).

Test Plan

This has seen a modicum of testing in a FreeBSD kernel
patched with 2ec2ba7e232d. The FreeBSD NFSv4 client
and server have also been patched to use them over NFSv4.

Further testing with versions of NFSv4 (Solaris) that already
supports named attributes will be done in the next few weeks.
(I might also find someone with a Mac who can test them
using their NFSv4 client.)

[rmacklem]

Fixed a bug when open'ng "." in a named attribute directory.

I also added a check to enforce the requirement for O_NAMEDATTR
be specified whenever open'ng either a named attribute directory
or a named attribute.

[rmacklem]

Cleanup up zfs_freebsd_lookup(), including adding some
comments to try and explain the named attribute case.

The only semantics change is using cn_lkflags instead
of LK_SHARED when locking the named attribute directory.