btrfs: fix the gfp_mask for the reada_zones radix tree

Commits cc8385b5 and 7ef70b4d added preallocation for the
reada radix trees and also switched them over to GFP_KERNEL for the
default gfp mask.

Since we're doing radix tree insertions under spinlocks, we need
to make sure the mask doesn't allow sleeping.  This fix keeps
the radix preallocation but switches back to the original gfp_mask.

Reported-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Chris Mason <clm@fb.com>