common: add helper function _flakey_drop_and_remount

common: add helper function _flakey_drop_and_remount

To avoid having many tests repeating the following pattern:

        _load_flakey_table $FLAKEY_DROP_WRITES
        _unmount_flakey

        _load_flakey_table $FLAKEY_ALLOW_WRITES
        _mount_flakey

add the helper function _flakey_drop_and_remount to remove
the existing duplicated code and serve as a shortcut.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>