erofs-utils: lib: fix sorting shared xattrs

erofs-utils: lib: fix sorting shared xattrs

The length of xattr_item->kvbuf is calculated by EROFS_XATTR_KVSIZE,
and the key part has a trailing '\0' before the value part. When qsort
compares two xattr_items, the key-value length should be calculated by
EROFS_XATTR_KVSIZE, and use memcmp instead of strncmp to avoid key-value
string being cut by '\0'.

Fixes: 5df285cf405d ("erofs-utils: lib: refactor extended attribute name prefixes")
Signed-off-by: Sheng Yong <shengyong@oppo.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20240913143542.3265071-1-shengyong@oppo.com
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>