fs: cifs: Replace _free_xid call in cifs_root_iget function

Modify end of cifs_root_iget function in fs/cifs/inode.c to call
free_xid(xid) instead of _free_xid(xid), thereby allowing debug
notification of this action when enabled.

Signed-off-by: Phillip Potter <phil@philpotter.co.uk>
Signed-off-by: Steve French <smfrench@gmail.com>