staging: lustre: ptlrpc: Replace kmem_cache_alloc with kmem_cache_zalloc

staging: lustre: ptlrpc: Replace kmem_cache_alloc with kmem_cache_zalloc

Use kmem_cache_zalloc instead of manually setting kmem_cache_alloc
with flag GFP_ZERO since kmem_alloc_zalloc sets allocated memory
to zero.

The Coccinelle semantic patch used to make this change is as
follows:
// <smpl>
@@
expression e,f;
@@
- kmem_cache_alloc(e, f |__GFP_ZERO)
+ kmem_cache_zalloc(e, f)
// </smpl>

Signed-off-by: Amitoj Kaur Chawla <amitoj1606@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>