LU-6142 kernel: use kmem_cache_zalloc as appropriate.

LU-6142 kernel: use kmem_cache_zalloc as appropriate.

Rather than passing __GFP_ZERO to kmem_cache_alloc(), or calling
memset(0) after the allocation, use kmem_cache_zalloc().

Also update spelling.txt to encourage use of kmem_cache_zalloc().

kmem_cache_zalloc() has been part of Linux since 2.6.17.

Test-Parameters: trivial
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: Ic4ccbe0e223121e54699f7667e35db14d0f0da70
Reviewed-on: https://review.whamcloud.com/38439
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Chris Horn <chris.horn@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>