afs: Fix non-setting of mtime when writing into mmap

The mtime on an inode needs to be updated when a write is made into an
mmap'ed section.  There are three ways in which this could be done: update
it when page_mkwrite is called, update it when a page is changed from dirty
to writeback or leave it to the server and fix the mtime up from the reply
to the StoreData RPC.

Found with the generic/215 xfstest.

Fixes: 1cf7a1518a ("afs: Implement shared-writeable mmap")
Signed-off-by: David Howells <dhowells@redhat.com>

fs/afs/write.c

@@ -844,6 +844,7 @@ vm_fault_t afs_page_mkwrite(struct vm_fault *vmf)
 			     vmf->page->index, priv);
 	SetPagePrivate(vmf->page);
 	set_page_private(vmf->page, priv);
+	file_update_time(file);
 
 	sb_end_pagefault(inode->i_sb);
 	return VM_FAULT_LOCKED;