treewide: Add __GFP_NOWARN to k.alloc calls with v.alloc fallbacks

Don't emit OOM warnings when k.alloc calls fail when
there there is a v.alloc immediately afterwards.

Converted a kmalloc/vmalloc with memset to kzalloc/vzalloc.

Signed-off-by: Joe Perches <joe@perches.com>
Acked-by: "Theodore Ts'o" <tytso@mit.edu>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>