Set FSType when constructing FS in luks::get_filesystem_support()

This is functionally identical, but is just to follow established coding
pattern [1] of specifying the FSType when constructing struct FS, rather
and setting it afterwards.  luks.cc was added after the aforementioned
commit, but was being developed in parallel so was created [2] following
the old coding pattern.

[1] 1a4cefb960
    Initialise all struct FS members

[2] 070d734e57
    Add busy detection of LUKS mapping (#760080)

src/luks.cc

@@ -42,8 +42,7 @@ const Glib::ustring & luks::get_custom_text( CUSTOM_TEXT ttype, int index ) cons
 
 FS luks::get_filesystem_support()
 {
-	FS fs;
-	fs.filesystem = FS_LUKS;
+	FS fs( FS_LUKS );
 
 	fs.busy = FS::EXTERNAL;
 	fs.read = FS::EXTERNAL;