cifs: fix sb->s_maxbytes so that it casts properly to a signed value

This off-by-one bug causes sendfile() to not work properly. When a task
calls sendfile() on a file on a CIFS filesystem, the syscall returns -1
and sets errno to EOVERFLOW.

do_sendfile uses s_maxbytes to verify the returned offset of the file.
The problem there is that this value is cast to a signed value (loff_t).
When this is done on the s_maxbytes value that cifs uses, it becomes
negative and the comparisons against it fail.

Even though s_maxbytes is an unsigned value, it seems that it's not OK
to set it in such a way that it'll end up negative when it's cast to a
signed value. These casts happen in other codepaths besides sendfile
too, but the VFS is a little hard to follow in this area and I can't
be sure if there are other bugs that this will fix.

It's not clear to me why s_maxbytes isn't just declared as loff_t in the
first place, but either way we still need to fix these values to make
sendfile work properly. This is also an opportunity to replace the magic
bit-shift values here with the standard #defines for this.

This fixes the reproducer program I have that does a sendfile and
will probably also fix the situation where apache is serving from a
CIFS share.

Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>