[CIFS] lseek polling returned stale EOF

Fixes Samba bug 4362

Discovered by Jeremy Allison

Clipper database polls on EOF via lseek and can get stale EOF
when file is open on different client

Signed-off-by: Jeremy Allison <jra@samba.org>
Signed-off-by: Steve French <sfrench@us.ibm.com>