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>