kernel/vm: Don't shrink commitment on precommit in VMAnonymousCache.

If we were "donated" some commitment (e.g. in copy_on_write_area)
then we don't want to drop it when pre-committing.

Fixes some commitments being too low in forked teams.

src/system/kernel/vm/VMAnonymousCache.cpp

@@ -734,9 +734,12 @@ VMAnonymousCache::Commit(off_t size, int priority)
 
 		// pre-commit some pages to make a later failure less probable
 		fHasPrecommitted = true;
-		uint32 precommitted = fPrecommittedPages * B_PAGE_SIZE;
+		uint32 precommitted = (fPrecommittedPages * B_PAGE_SIZE);
 		if (size > precommitted)
 			size = precommitted;
+
+		// pre-commit should not shrink existing commitment
+		size += committed_size;
 	}
 
 	return _Commit(size, priority);