s3:libsmb: Store file allocation size in struct file_info.

We get this already, from SMB2_FIND_ID_BOTH_DIRECTORY_INFO
so return it.

For SMB1 leave this as zero so callers know it hasn't
been initialized.

Signed-off-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>
