tsn: replace GFP_KERNEL with GFP_ATOMIC to avoid sleeping in atomic context

When running qbv test of TSN feature in RT kernel, there is below call trace
BUG: sleeping function called from invalid context at include/linux/sched/mm.h:229
in_atomic(): 0, irqs_disabled(): 0, non_block: 0, pid: 502, name: tsntool
preempt_count: 0, expected: 0
RCU nest depth: 1, expected: 0
CPU: 0 PID: 502 Comm: tsntool Not tainted 5.15.65-rt48-yocto-preempt-rt #1
Hardware name: LS1028A RDB Board (DT)
Call trace:
 dump_backtrace+0x0/0x1a0
 ......
__might_resched+0x160/0x1c0
__might_sleep+0x58/0xb0
 kmem_cache_alloc+0x260/0x310
 __alloc_skb+0x1a8/0x1d0
 tsn_event+0x58/0x230
 atomic_notifier_call_chain+0x6c/0x9c
 call_tsn_notifiers+0x30/0x40
 enetc_qbv_set+0x2e8/0x5fc
 cmd_qbv_set.isra.0+0x1d4/0x300
 tsn_qbv_set+0x2c/0x44
 ......
 el0t_64_sync+0x1a0/0x1a4
So, replace GFP_KERNEL with GFP_ATOMIC to avoid sleeping
in atomic context.

Signed-off-by: Meng Li <Meng.Li@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>