kernel/user_mutex: Panic instead of failing due to OOM.

This is not an expected failure condition for most applications,
so try to avoid propagating it to userspace.

(If necessary, it should be possible to avoid even the possibility
of this by just allocating necessary datastructures upfront
instead of "lazily". But that seems unnecessary at the moment.)

src/system/kernel/locks/user_mutex.cpp

@@ -284,8 +284,10 @@ get_user_mutex_entry(struct user_mutex_context* context,
 	}
 
 	entry = new(std::nothrow) UserMutexEntry;
-	if (entry == NULL)
+	if (entry == NULL) {
+		panic("UserMutexEntry allocation failed!");
 		return entry;
+	}
 
 	entry->address = address;
 	entry->ref_count = 1;
@@ -469,6 +471,7 @@ struct UserMutexContextFetcher {
 		if (!fShared) {
 			fContext = get_team_user_mutex_context();
 			if (fContext == NULL) {
+				panic("UserMutexContext allocation failed!");
 				fInitStatus = B_NO_MEMORY;
 				return;
 			}