btrfs: avoid blocking when allocating context for nowait dio read/write

btrfs: avoid blocking when allocating context for nowait dio read/write

When doing a NOWAIT direct IO read/write, we allocate a context object
(struct btrfs_dio_data) with GFP_NOFS, which can result in blocking
waiting for memory allocation (GFP_NOFS is __GFP_RECLAIM | __GFP_IO).
This is undesirable for the NOWAIT semantics, so do the allocation with
GFP_NOWAIT if we are serving a NOWAIT request and if the allocation fails
return -EAGAIN, so that the caller can fallback to a blocking context and
retry with a non-blocking write.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>