tools/disktool: Don't memcmp() with a block of zeros
tools/disktool: Don't memcmp() with a block of zeros

- No longer call memcmp() to compare a buffer to a pre-allocated zeroed
block; instead, compare individual words/bytes of the buffer to zero.
This improves performance when importing from /dev/zero by roughly a
factor of two.

cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr