fs/buffer.c: support buffer cache allocations with gfp modifiers

fs/buffer.c: support buffer cache allocations with gfp modifiers

commit 3b5e6454aaf6b4439b19400d8365e2ec2d24e411 upstream.

A buffer cache is allocated from movable area because it is referred
for a while and released soon.  But some filesystems are taking buffer
cache for a long time and it can disturb page migration.

New APIs are introduced to allocate buffer cache with user specific
flag.  *_gfp APIs are for user want to set page allocation flag for
page cache allocation.  And *_unmovable APIs are for the user wants to
allocate page cache from non-movable area.

Signed-off-by: Gioh Kim <gioh.kim@lge.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>
[bwh: Prerequisite for "bufferhead: Add _gfp version for sb_getblk()".
 Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>