sa_find_sizes() may compute wrong SA header size

Under the right conditions sa_find_sizes() will compute an incorrect
size of the system attribute (SA) header. This causes a failed assertion
when the SA_HDR_SIZE_MATCH_LAYOUT() test returns false, and may lead
to corruption of SA data.

The bug presents itself when there are more than two variable-length SAs
of just the right size to fit in the bonus buffer of a dnode. The
existing logic fails to account for the SA header space needed to store
the sizes of all the variable-length SAs.

A reproducer was possible on Linux by setting the xattr=sa dataset
property and storing xattrs on symbolic links (Issue #1648). Note the
corrupt link target name:

$ zfs set xattr=sa tank/fish
$ cd /tank/fish
$ ln -fs 12345678901234567 link
$ setfattr -n trusted.0000000000000000000 -v 0x000000000000000000000000 -h link
$ setfattr -n trusted.1111111111111111111 -v 0x000000000000000000000000 -h link
$ ls -l link
lrwxrwxrwx 1 root root 17 Dec 6 15:40 link -> 90123456701234567

Commit 6a7c0ccca44ad02c476a111d8f7911fc8b12fff7 worked around this bug
by forcing xattr's on symlinks to be stored in directory format. This
change implements a proper fix, so the workaround can now be reverted.

The reference link below contains a reproducer for FreeBSD.

References:

http://lists.open-zfs.org/pipermail/developer/2013-November/000306.html

Ported-by: Ned Bass <bass6@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1890