pNFS/flexfiles: Fix an Oopsable typo in ff_mirror_match_fh()

Jeff reports seeing an Oops in ff_layout_alloc_lseg. Turns out
copy+paste has played cruel tricks on a nested loop.

Reported-by: Jeff Layton <jeff.layton@primarydata.com>
Cc: stable@vger.kernel.org # 4.3+
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>