knfsd: exportfs: split out reconnecting a dentry from find_exported_dentry

There's a clear subfunctionality of reconnecting a given dentry to the main
dentry tree in find_exported_dentry, that can be called both for the dentry
we're looking for or it's parent directory.

This patch splits the subfunctionality out into a separate helper to make the
code more readable and document it's intent.  As a nice side-optimization we
can avoid getting a superfluous dentry reference count in the case we need to
reconnect a directory on it's own.

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>