e2fsck: add optimization for large, fragmented sparse files

e2fsck: add optimization for large, fragmented sparse files

The code which checks for overlapping logical blocks in an extent tree
is O(h*e) in time, where h is the number of holes in the file, and e
is the number of extents in the file.  So a file with a large number
of holes can take e2fsck a long time process.  Optimize this taking
advantage of the fact the vast majority of the time, region_allocate()
is called with increasing logical block numbers, so we are almost
always append onto the end of the region list.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>