commit c45653c3 upstream.

Switch ext4 to using sb_getblk_gfp with GFP_NOFS added to fix possible
deadlocks in the page writeback path.
Signed-off-by: Nikolay Borisov <kernel@kyup.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
[lizf: Backported to 3.4: adjust context]
Signed-off-by: Zefan Li <lizefan@huawei.com>