cifs: change SMB2_OP_SET_EOF to use compounding

This changes SMB2_OP_SET_EOF to use compounding in some situations.
This is part of the path based API to truncate a file.
Most of the time this will however not be invoked for SMB2 since
cifs_set_file_size() will as far as I can tell almost always just
open the file synchronously and switch to the handle based truncate
code path, thus bypassing the compounding we add here.

Rewriting cifs_set_file_size() and make that whole pile of code more
compounding friendly, and also easier to read and understand, is a
different project though and not for this patch.

Signed-off-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>