smbd: set fsp_flags.is_fsa to true on printer file handles

smbd: set fsp_flags.is_fsa to true on printer file handles

Printer file handles went through SMB_VFS_CREATE_FILE() and are network
callable, so it makes sense to set this on them.

This ensures that check_access_fsp() doesn't take the codepath calling
smbd_check_access_rights_fsp(), but just checks the request rights from
fsp->access_mask.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13688

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>