gfs2: Don't update inode timestamps for direct writes

During direct reads and writes, the caller is holding the inode glock in
deferred mode, which doesn't allow metadata updates.  However, a previous
change caused callers to update the inode modification time before carrying out
direct writes, which caused the inode glock to be converted to exclusive mode
for the timestamp update, only to be immediately converted back to deferred
mode for the direct write.  This locks out other direct readers and writers
and wreaks havoc on performance.

Fix that by reverting to not updating the inode modification time for direct
writes.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>