s390/ctcm: Use explicit allocation mask in ctcmpc_unpack_skb().

gfp_type() uses in_interrupt() to figure out the correct GFP mask.

The usage of in_interrupt() in drivers is phased out and Linus clearly
requested that code which changes behaviour depending on context should
either be separated or the context be conveyed in an argument passed by the
caller, which usually knows the context.

The call chain of ctcmpc_unpack_skb():
ctcmpc_bh()
 -> ctcmpc_unpack_skb()

ctcmpc_bh() is a tasklet handler so GFP_ATOMIC is needed.

Use GFP_ATOMIC as allocation type in ctcmpc_unpack_skb().

Reviewed-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>