Post-push fix for 10.5+.

The fix influence MDEV-14479. Before the fix
lock_rec_convert_impl_to_expl() did not create explicit lock if caller's
transaction owns found implicit lock(see MDEV-14479 for details). After the fix
lock_rec_convert_impl_to_expl() can create explicit lock under the above
conditions if the requested lock mode is not LOCK_REC_NOT_GAP. And that
is why we need to check if the table is X-locked before
lock_rec_convert_impl_to_expl() call.