um: net: replace GFP_KERNEL with GFP_ATOMIC when spinlock is held

since GFP_KERNEL with GFP_ATOMIC while spinlock is held,
as code while holding a spinlock should be atomic.
GFP_KERNEL may sleep and can cause deadlock,
where as GFP_ATOMIC may fail but certainly avoids deadlockdex f70dd54..d898f6c 100644

Signed-off-by: Saurabh Sengar <saurabh.truth@gmail.com>
Signed-off-by: Richard Weinberger <richard@nod.at>