misc/pvpanic-mmio: Use GFP_KERNEL instead of GFP_ATOMIC

There is no need to use GFP_ATOMIC in a probe function. Use GFP_KERNEL
instead.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/58cc7f12535a796a0ef1a699bcba61e45ab8a2ad.1621665058.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>