libext2fs: find/alloc a range of empty blocks

libext2fs: find/alloc a range of empty blocks

Provide a function that, given a goal pblk and a range, will try to
find a run of free blocks to satisfy the allocation.  By default the
function will look anywhere in the filesystem for the run, though this
can be constrained with optional flags.  One flag indicates that the
range must start at the goal block; the other flag indicates that we
should not return a range shorter than len.

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