btrfs: allocate btrfs_io_context without GFP_NOFAIL

The __GFP_NOFAIL flag could loop indefinitely when allocation memory in
alloc_btrfs_io_context. The callers starting from __btrfs_map_block
already handle errors so it's safe to drop the flag.
Signed-off-by: Li zeming <zeming@nfschina.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/volumes.c

@@ -5899,7 +5899,10 @@ static struct btrfs_io_context *alloc_btrfs_io_context(struct btrfs_fs_info *fs_
 		 * and the stripes.
 		 */
 		sizeof(u64) * (total_stripes),
-		GFP_NOFS|__GFP_NOFAIL);
+		GFP_NOFS);
+
+	if (!bioc)
+		return NULL;
 
 	refcount_set(&bioc->refs, 1);