crypto: cavium/nitrox - Replace GFP_ATOMIC with GFP_KERNEL in crypto_alloc_context()

crypto_alloc_context() is only called by nitrox_skcipher_init(), which is
never called in atomic context.

crypto_alloc_context() calls dma_pool_alloc() with GFP_ATOMIC,
which is not necessary.
GFP_ATOMIC can be replaced with GFP_KERNEL.

This is found by a static analysis tool named DCNS written by myself.
I also manually check the kernel code before reporting it.

Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>