fix(btrfs): allocate the swapfile with fallocate instead of dd

On a btrfs root the swapfile was written with dd from /dev/zero, so a
swapfile the size of the RAM cost tens of GiB of writes and stalled
every other writer on the filesystem. A caller that bounds the run, such
as an async Ansible task, killed dd halfway, and the next run found the
wrong size and started over. btrfs accepts a preallocated NOCOW
swapfile and the swap directory already carries +C, so every filesystem
now allocates with fallocate.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

tests/e2e/run.sh

@@ -21,7 +21,7 @@ case "${SWAPFORGE}" in
 esac
 echo "ok: resolved from the installed package, not the source tree"
 
-for binary in findmnt swapoff chmod mkswap swapon fallocate chattr btrfs dd; do
+for binary in findmnt swapoff fallocate chmod mkswap swapon chattr btrfs; do
     command -v "${binary}" >/dev/null || fail "${binary} is missing from the image"
 done
 echo "ok: every declared binary is present"