fs: export inode_to_bdi and use it in favor of mapping->backing_dev_info

Now that we got rid of the bdi abuse on character devices we can always use
sb->s_bdi to get at the backing_dev_info for a file, except for the block
device special case.  Export inode_to_bdi and replace uses of
mapping->backing_dev_info with it to prepare for the removal of
mapping->backing_dev_info.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@fb.com>

diff --cc fs/btrfs/file.c
Simple merge

diff --cc fs/ceph/file.c
Simple merge

diff --cc fs/ext2/ialloc.c
Simple merge

diff --cc fs/ext4/super.c
Simple merge

diff --cc fs/fs-writeback.c
Simple merge

diff --cc fs/fuse/file.c
Simple merge

diff --cc fs/gfs2/aops.c
Simple merge

diff --cc fs/gfs2/super.c
Simple merge

diff --cc fs/nfs/filelayout/filelayout.c
Simple merge

diff --cc fs/nfs/write.c
Simple merge

diff --cc fs/ntfs/file.c
Simple merge

diff --cc fs/ocfs2/file.c
Simple merge

diff --cc fs/xfs/xfs_file.c
Simple merge

diff --cc include/linux/backing-dev.h
Simple merge

diff --cc include/trace/events/writeback.h
Simple merge

diff --cc mm/fadvise.c
Simple merge

diff --cc mm/filemap.c
Simple merge

diff --cc mm/filemap_xip.c
Simple merge

diff --cc mm/page-writeback.c
Simple merge

diff --cc mm/readahead.c
Simple merge

diff --cc mm/truncate.c
Simple merge

diff --cc mm/vmscan.c
Simple merge