If 'num_pages' were ever 1 more than a multiple of 8 (32bit platforms)
or of 16 (64 bit platforms).  filemap_attr would be allocated one
'unsigned long' shorter than required.  We need a round-up in there.
Signed-off-by: Neil Brown <neilb@suse.de>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>