libmount: try read-only mount on write-protected superblock too

libmount: try read-only mount on write-protected superblock too

The classic mount(8) behavior is to try read-only on write-protected devices
if the first mount syscall attempt ends with EACCES.

It seems we can implement this feature also for EBUSY if the same mount source
is already mounted with "ro" superblock option.

The typical use-case is iso image (always read-only) mounted on two places.

Addresses: https://bugzilla.redhat.com/show_bug.cgi?id=1809124
Signed-off-by: Karel Zak <kzak@redhat.com>