s3: smbd: Remove the handling of offset == -1 in real_write_file().

s3: smbd: Remove the handling of offset == -1 in real_write_file().

All SMB1/2/3 offsets over the wire are absolute.

The only caller with offset == -1 is on a print-spool file
in reply_printwrite(), and write_file() redirects this
to print_spool_write(), which correctly handles the -1.

Signed-off-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>