reserve_region_with_split: Fix GFP_KERNEL usage under spinlock

This one apparently doesn't generate any warnings, because the function
is only used during system bootup, when the warnings are disabled.  But
it's still very wrong.

The __reserve_region_with_split() function is called with the
resource_lock held for writing, so it must only ever do GFP_ATOMIC
allocations.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>