erofs-utils: lib: fix errors when building xattrs

EOPNOTSUPP could be reported by llistxattr() and mkfs.erofs could
fail out due to the following error:

<E> erofs: llistxattr to get the size of names for xxxxx failed
<E> erofs: 	Could not format the device : [Error 95] Operation not supported

Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20230309112630.74230-3-hsiangkao@linux.alibaba.com
1 file changed