ocfs2: clean up bh null checks

If we know a buffer_head is non-null, then brelse() is unnecessary and
put_bh() can be used instead. Also, an explicit check for NULL is
unnecessary when using brelse(). This patch only covers buffer_head_io.c and
resize.c, which have recently added code which exhibits this problem.

Signed-off-by: Mark Fasheh <mark.fasheh@oracle.com>