kernel.h: Silence sparse warning in lower_32_bits

I keep getting sparse warnings in crypto such as:

  CHECK   drivers/crypto/ccree/cc_hash.c
   drivers/crypto/ccree/cc_hash.c:49:9: warning: cast truncates bits from constant value (47b5481dbefa4fa4 becomes befa4fa4)
   drivers/crypto/ccree/cc_hash.c:49:26: warning: cast truncates bits from constant value (db0c2e0d64f98fa7 becomes 64f98fa7)
   [.. many more ..]

This patch removes the warning by adding a mask to keep sparse
happy.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>