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>