Revert "rhashtable: Use __vmalloc with GFP_ATOMIC for table allocation"

This reverts commit d3716f18

.

vmalloc cannot be used in BH disabled contexts, even
with GFP_ATOMIC.  And we certainly want to support
rhashtable users inserting entries with software
interrupts disabled.

Signed-off-by: David S. Miller <davem@davemloft.net>