mds/MDLog: use committed seq instead of committing seq

mds/MDLog: use committed seq instead of committing seq

Since commit 9242ce90130 it won't allow multiple open file table
commits to be submit and will be worked sequentially. So whenever
is_any_committing() is false the committing seq will always equal
to committed seq.

Signed-off-by: Xiubo Li <xiubli@redhat.com>
(cherry picked from commit fb2477b29721a87a5f5f3c0b8a687ccbccf2b9c8)