btrfs: fix the gfp_mask for the reada_zones radix tree

btrfs: fix the gfp_mask for the reada_zones radix tree

Commits 2f5a36ded957 and 3e9a407a6101eb 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>