udf: Fix use of check_add_overflow() with mixed type arguments

Commit ebbe26fd "udf: Avoid excessive partition lengths"
introduced a use of check_add_overflow() with argument types u32,
size_t, and u32 *.

This was backported to the 5.x stable branches, where in 64-bit
configurations it results in a build error (with older compilers) or a
warning.  Before commit d219d2a9 "overflow: Allow mixed type
arguments", which went into Linux 6.1, mixed type arguments are not
supported.  That cannot be backported to 5.4 or 5.10 as it would raise
the minimum compiler version for these kernel versions.

Add a cast to make the argument types compatible.

Fixes: 1497a448 ("udf: Avoid excessive partition lengths")
Fixes: 55196637 ("udf: Avoid excessive partition lengths")
Signed-off-by: Ben Hutchings <benh@debian.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>