exportfs: move most of reconnect_path to helper function

Also replace 3 easily-confused three-letter acronyms by more helpful
variable names.

Just cleanup, no change in functionality, with one exception: the
dentry_connected() check in the "out_reconnected" case will now only
check the ancestors of the current dentry instead of checking all the
way from target_dir.  Since we've already verified connectivity up to
this dentry, that should be sufficient.

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>