libmount: Fix access check for utab in context

libmount: Fix access check for utab in context

The function mnt_has_regular_utab() properly detects that the utab is
not writable, but this is ignored by the high-level context API. As a
result, the library later attempts to update the file and ends up with
a warning in mount(8):

 $ mkdir sys
 $ unshare --map-root-user --mount
 $ mount --rbind /sys sys
 $ umount --lazy sys; echo $?
 umount: /home/user/sys: filesystem was unmounted, but failed to update userspace mount table.
 16

In this case, the utab should be ignored.

Fixes: https://github.com/util-linux/util-linux/issues/2981
Signed-off-by: Karel Zak <kzak@redhat.com>