knfsd: exportfs: move acceptable check into find_acceptable_alias

All callers of find_acceptable_alias check if the current dentry is acceptable
before looking for other acceptable aliases using find_acceptable_alias.  Move
the check into find_acceptable_alias to make the code a little more dense and
add a comment to find_acceptable_alias that documents its intent.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>