libext2fs: implement fallocate

Create a library function to perform fallocation on arbitrary files.
This is a bit more intense than Ted's original mk_hugefiles
implementation since we have to honor any blocks that may already be
allocated to the file.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
3 files changed