knfsd: exportfs: untangle ISDIR logic in find_exported_dentry

Rework some logic in find_exported_dentry so that we only have a single
S_ISDIR check and logic that makes clear to the reader what we're really doing
here.

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>