common: fix _require_xfs_io_command pwrite -A for various blocksizes

In this predicate, we should test an atomic write of the minimum
supported size, not just 4k.  This fixes a problem where none of the
atomic write tests actually run on a 32k-fsblock xfs because you can't
do a sub-fsblock atomic write.

Cc: <fstests@vger.kernel.org> # v2025.04.13
Fixes: d90ee3b6496346 ("generic: add a test for atomic writes")
Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>