ext4: remove unnecessary NULL pointer check

brelse() and ext4_journal_force_commit() are both inlined and able
to handle NULL.

Signed-off-by: Guo Chao <yan@linux.vnet.ibm.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>