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

[ Upstream commit bb413489288e4e457353bac513fddb6330d245ca ]

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: 1cf7a151 ("afs: Implement shared-writeable mmap")
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>