fs: erofs: xattr.c change kzalloc to kcalloc

Refactor xattr.c to use kcalloc instead of kzalloc when multiplying
allocation size by count. This refactor prevents unintentional
memory overflows. Discovered by checkpatch.pl.

Signed-off-by: Ethan Carter Edwards <ethan@ethancedwards.com>
Link: https://lore.kernel.org/r/i3CLJhMELKzBJr3DaRyv-hP_4m-3Twx0sgBWXW6naZlMtHrIeWr93xOFshX8qZHDrJeSjHMTiUOh8JmBZ9v0AB-S1lIYM_d-vasSRlsF_s4=@ethancedwards.com


Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>