When MariaDB is built with PERFORMANCE_SCHEMA support enabled
and with futex-based rw-locks (not srw_lock_), we were unnecessarily
releasing and reacquiring lock.writer in srw_lock_impl::psi_wr_lock()
and ssux_lock::psi_wr_lock().

If there is a conflict with rd_lock(), let us hold the lock.writer
and execute u_wr_upgrade() to wait for rd_unlock().

Reviewed by: Debarun Banerjee
Tested by: Matthias Leich