[CIFS] fix regression in cifs_write_begin/cifs_write_end

The conversion to write_begin/write_end interfaces had a bug where we
were passing a bad parameter to cifs_readpage_worker. Rather than
passing the page offset of the start of the write, we needed to pass the
offset of the beginning of the page. This was reliably showing up as
data corruption in the fsx-linux test from LTP.

It also became evident that this code was occasionally doing unnecessary
read calls. Optimize those away by using the PG_checked flag to indicate
that the unwritten part of the page has been initialized.

CC: Nick Piggin <npiggin@suse.de>
Acked-by: Dave Kleikamp <shaggy@us.ibm.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>