FreeBSD: Implement xattr=sa

FreeBSD historically has not cared about the xattr property; it was
always treated as xattr=on. With xattr=on, xattrs are stored as files
in a hidden xattr directory. With xattr=sa, xattrs are stored as
system attributes and get cached in nvlists during xattr operations.
This makes SA xattrs simpler and more efficient to manipulate. FreeBSD
needs to implement the SA xattr operations for feature parity with
Linux and to ensure that SA xattrs are accessible when migrated or
replicated from Linux.

Following the example set by Linux, refactor our existing extattr vnops
to split off the parts handling dir style xattrs, and add the
corresponding SA handling parts.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #11997