fsperf: Fix for perl 5.18

On perl 5.18, "bignum" dosn't seems to handle undef as 0, like following.

	$ perl -Mbignum -e 'my $x; $x += 3;'
	Can't call method "isa" on an undefined value at /usr/share/perl/5.18/Math/BigInt.pm line 1131.

So, this patch explicitly initialize values, and to do it, adds helpers,

[helpers handles undefined as 0]

num_add(a, b)  - add b to a
num_sub(a, b)  - sub b from a
num_max(a, b)  - set max(a, b) to a
num_min(a, b)  - set min(a, b) to a

Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>