Buffer: Use 256GB mmap size instead of MaxInt64

[jarifibrahim]

MaxInt64 is 9.2 Exabyte and the test fails with cannot allocate memory on my computer.
This PR also fixes the build (it is failing on master).

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This change is 

[martinmr]

Reviewed 1 of 1 files at r1.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @anurags92, @jarifibrahim, @karlmcguire, and @manishrjain)

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z/buffer.go, line 73 at r1 (raw file):

// Newbuffer is a helper utility, which creates a virtually unlimited Buffer in UseCalloc mode.
func NewBuffer(sz int) *Buffer {
	buf, err := NewBufferWith(sz, math.MaxInt32, UseCalloc)

so this is now 2GB. not virtually unlimited anymore. Is that ok?

[jarifibrahim]

Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @anurags92, @karlmcguire, @manishrjain, and @martinmr)

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z/buffer.go, line 73 at r1 (raw file):

Previously, martinmr (Martin Martinez Rivera) wrote…

so this is now 2GB. not virtually unlimited anymore. Is that ok?

I can make it 100*math.maxInt32 or something. math.MaxInt64 crashes on my computer.