kernel/cache: Acquire store references in PrecacheIO.

If the file in question is deleted while we are performing asynchronous
I/O, and there is nothing else left with a reference to it, then the
underlying vnode could wind up getting deleted from under us, since
the cache does not maintain a reference to the store unless there
are active mappings (areas) associated with the cache.

Should fix #19122. At least, I couldn't find any other places
that performed asynchronous I/O without properly acquiring and
releasing references to the underlying vnode besides this one.

src/system/kernel/cache/file_cache.cpp

@@ -128,6 +128,7 @@ PrecacheIO::PrecacheIO(file_cache_ref* ref, off_t offset, generic_size_t size)
 {
 	fPageCount = (size + B_PAGE_SIZE - 1) / B_PAGE_SIZE;
 	fCache->AcquireRefLocked();
+	fCache->AcquireStoreRef();
 }
 
 
@@ -135,6 +136,7 @@ PrecacheIO::~PrecacheIO()
 {
 	delete[] fPages;
 	delete[] fVecs;
+	fCache->ReleaseStoreRef();
 	fCache->ReleaseRefLocked();
 }