btrfs: use GFP_KERNEL in btrfs_quota_enable

We don't need to use GFP_NOFS here as this is called from ioctls an the
only lock held is the subvol_sem, which is of a high level and protects
creation/renames/deletion and is never held in the writeout paths.

Signed-off-by: David Sterba <dsterba@suse.com>