Bring in reallocblk to ext2fs.

Bring in reallocblk to ext2fs.

The feature has been standard for a while in UFS as a means to reduce
fragmentation, therefore maintaining consistent performance with
filesystem aging. This is also very similar to what ext4 calls
"delayed allocation".

In his 2010 GSoC, Zheng Liu ported and benchmarked the missing
FANCY_REALLOC code to find more consistent performance improvements than
with the preallocation approach.

PR:		159233
Author:		Zheng Liu <gnehzuil AT SPAMFREE gmail DOT com>
Sponsored by:	Google Inc.
Approved by:	jhb (mentor)
MFC after:	2 weeks