fuse: improve error handling in fuse_ll_getattr

fuse: improve error handling in fuse_ll_getattr

Currently, fuse_ll_getattr always returns ENOENT when ll_getattr fails,
which masks the actual error conditions. This makes debugging and error
handling difficult for applications using ceph-fuse.

This change preserves the original error code from ll_getattr and converts
it to the appropriate system error code using get_sys_errno(), allowing
applications to receive accurate error information.

Signed-off-by: izxl007 <zeng.zheng@zte.com.cn>