Handle cas failure when the compare succeeds

When locking a priority inherit mutex we perform a compare and swap
operation to try and acquire the mutex. This may fail even when the
compare succeeds.

Check and handle this case.

PR:		263825
Reviewed by:	kib, markj
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35150

sys/kern/kern_umtx.c

@@ -2262,6 +2262,17 @@ do_lock_pi(struct thread *td, struct umutex *m, uint32_t flags,
 			break;
 		}
 
+		/*
+		 * Nobody owns it, but the acquire failed. This can happen
+		 * with ll/sc atomics.
+		 */
+		if (owner == UMUTEX_UNOWNED) {
+			error = thread_check_susp(td, true);
+			if (error != 0)
+				break;
+			continue;
+		}
+
 		/*
 		 * Avoid overwriting a possible error from sleep due
 		 * to the pending signal with suspension check result.