sysfs: Do not return POSIX ACL xattrs via listxattr

sysfs: Do not return POSIX ACL xattrs via listxattr

Commit 786534b92f3c introduced a regression that caused listxattr to
return the POSIX ACL attribute names even though sysfs doesn't support
POSIX ACLs.  This happens because simple_xattr_list checks for NULL
i_acl / i_default_acl, but inode_init_always initializes those fields
to ACL_NOT_CACHED ((void *)-1).  For example:
    $ getfattr -m- -d /sys
    /sys: system.posix_acl_access: Operation not supported
    /sys: system.posix_acl_default: Operation not supported
Fix this in simple_xattr_list by checking if the filesystem supports POSIX ACLs.

Fixes: 786534b92f3c ("tmpfs: listxattr should include POSIX ACL xattrs")
Reported-by:  Marc Aurèle La France <[email protected]>
Tested-by: Marc Aurèle La France <[email protected]>
Signed-off-by: Andreas Gruenbacher <[email protected]>
Cc: [email protected] # v4.5+
Signed-off-by: Al Viro <[email protected]>