fallocate: dig holes only in data extents

fallocate: dig holes only in data extents

Based on patch from Vaclav Dolezal <vdolezal@redhat.com>, this
implementation is less invasive.

The patch adds a new while() for pread() call (so diff is mostly code
indention). The pread() is called for a real data only (addressed by
'off' and 'end') and we use SEEK_{DATA,HOLE} before the pread() to
skip already existing holes. The variables 'file_off' and 'file_end'
addresses area in the file as specified on fallocate command line.

Test:

$ truncate -s 10G testfile
$ dd if=/dev/zero of=testfile count=10 bs=1M conv=notrunc

old version:

	$ time /usr/bin/fallocate --dig-holes --verbose testfile
	testfile: 10 GiB (10737418240 bytes) converted to sparse holes.

	real	0m3.013s
	user	0m0.700s
	sys	0m2.304s

new version:

	$ time ./fallocate --dig-holes --verbose testfile
	testfile: 10 MiB (10485760 bytes) converted to sparse holes.

	real	0m0.026s
	user	0m0.002s
	sys	0m0.004s

The old version scans all file.

The change has minimal overhead for files without holes.

Addresses: https://github.com/karelzak/util-linux/issues/421
Co-Author: Vaclav Dolezal <vdolezal@redhat.com>
Signed-off-by: Karel Zak <kzak@redhat.com>