generic/377: filter out xattrs that don't start with 'user.'

generic/377: filter out xattrs that don't start with 'user.'

Most hosts that I've been testing on seem to display security.selinux in
listxattr. 377.out doesn't account for that so it routinely fails for me
in testing.

When testing the output of listxattr in generic/377, filter out any xattr
names that don't start with 'user.'. That should help ensure consistent
output on SELinux-enabled hosts.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>