exportfs: fix quadratic behavior in filehandle lookup

Suppose we're given the filehandle for a directory whose closest
ancestor in the dcache is its Nth ancestor.

The main loop in reconnect_path searches for an IS_ROOT ancestor of
target_dir, reconnects that ancestor to its parent, then recommences the
search for an IS_ROOT ancestor from target_dir.

This behavior is quadratic in N.  And there's really no need to restart
the search from target_dir each time: once a directory has been looked
up, it won't become IS_ROOT again.  So instead of starting from
target_dir each time, we can continue where we left off.

This simplifies the code and improves performance on very deep directory
heirachies.  (I can't think of any reason anyone should need heirarchies
a hundred or more deep, but the performance improvement may be valuable
if only to limit damage in case of abuse.)

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>