ext4: don't update the superblock in ext4_statfs()

commit a71ce8c6 updated ext4_statfs()
to update the on-disk superblock counters, but modified this buffer
directly without any journaling of the change.  This is one of the
accesses that was causing the crc errors in journal replay as seen in
kernel.org bugzilla #14354.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: stable@kernel.org