libmount: fix mnt_context_next_remount()

libmount: fix mnt_context_next_remount()

The current implementation works, but the remount operation is done in
the cloned context and the original context (and calling application)
has no information about the final status/errors. This is mistake.

This new implementation works like mnt_context_next_mount(), it means
the same context (as used by application) is reused for all remounts.
The original setting is restored by mnt_context_apply_template().

Signed-off-by: Karel Zak <kzak@redhat.com>