snapshot_test: Fix overflow in statvfs arithmetics

Operations on (unsigned long) f_bsize and f_bfree are correctly handled
on 64 bit architectures, but when switching to 32 bit, the
multiplication between the two could result in an overflow, causing the
test to crash.
Fix by using a uint64_t operand, hinting the compiler to handle the
whole multiplication with 64 bit operands.

Bug: 154355449
Bug: 148889015
Test: vts_libsnapshot_test (32 bit)
Signed-off-by: Alessio Balsini <balsini@google.com>
Change-Id: Ief5c03ff7954c4a3e8597ef6e7df467f59428877