fsx: don't print messages when atomic writes are explicitly disabled

The user knows they disabled atomic writes, no need to clutter up the
output with a message that also causes golden output failures if they
start fstests with FSX_AVOID='-a'.

Cc: fstests@vger.kernel.org # v2025.10.20
Fixes: e2bc78e9d340a6 ("ltp/fsx.c: Add atomic writes support to fsx")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Zorro Lang <zlang@kernel.org>