hammer: fix terminology of "large block"

hammer: fix terminology of "large block"

This cleanup patch changes terminology "large block" to "big block".

- Both "large block" and "big block" are widely used in hammer source from
kernel to userspace, however these two refer to the same data structure which
is a 8MB sized chunk within low level blockmapped storage layer.

- The original design document https://www.dragonflybsd.org/hammer/hammer.pdf
uses big block for this data structure. Having two expressions in its
implementation is confusing and makes grep difficult.

Closes: #2782