s390/ctcm: Use GFP_KERNEL in add_channel().

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 memory allocation of `ch' a few lines above is using GFP_KERNEL,
also an allocation a few lines later is using GFP_KERNEL.

Use GFP_KERNEL for the memory allocation.

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>