btrfs: fix up misleading GFP_NOFS usage in btrfs_releasepage

b335b003 ("Btrfs: Avoid using __GFP_HIGHMEM with slab allocator")
has reduced the allocation mask in btrfs_releasepage to GFP_NOFS just
to prevent from giving an unappropriate gfp mask to the slab allocator
deeper down the callchain (in alloc_extent_state). This is wrong for
two reasons a) GFP_NOFS might be just too restrictive for the calling
context b) it is better to tweak the gfp mask down when it needs that.

So just remove the mask tweaking from btrfs_releasepage and move it
down to alloc_extent_state where it is needed.

Signed-off-by: Michal Hocko <mhocko@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>