tests: Fix test/misc/swaplabel failure due to change in mkswap behaviour.

tests: Fix test/misc/swaplabel failure due to change in mkswap behaviour.

mkswap now warns if the image file has holes. If fallocate is used to create the
file, use POSIX semantics to ensure the file has no holes.

This fixes the test failure

misc: swaplabel                      ... FAILED (misc/swaplabel)
========= script: /build/util-linux-2.37.2/tests/ts/misc/swaplabel =================
================= OUTPUT =====================
     1  Setting up swapspace version 1, size = 9 pages (9xPGSZ bytes)
     2  LABEL=1234567890abcde, UUID=12345678-abcd-abcd-abcd-1234567890ab
     3  LABEL: 1234567890abcde
     4  UUID:  12345678-abcd-abcd-abcd-1234567890ab
================= EXPECTED ===================
     1  Setting up swapspace version 1, size = 9 pages (9xPGSZ bytes)
     2  LABEL=1234567890abcde, UUID=12345678-abcd-abcd-abcd-1234567890ab
     3  LABEL: 1234567890abcde
     4  UUID:  12345678-abcd-abcd-abcd-1234567890ab
================= O/E diff ===================
==============================================

The additional error appears in swaplabel.err:

 mkswap: <swapfile> contains holes or other unsupported extents.
         This swap file can be rejected by kernel on swap activation!
         Use --verbose for more details.

[zeha@debian.org: this appears to be important/required when building
inside cowbuilder, on an ext3 filesystem.]

Signed-off-by: Mark Hindley <mark@hindley.org.uk>