libmount: preserve reused loop devices and request direct I/O

Hi Karel,

This revision addresses your review of PR #4646. The reused-device fix is now separate from the direct-I/O option.

Changes in v2:
- Add a specific diagnostic for direct-I/O mode conflicts.
- Use ul_strtobool() and the test-suite cleanup helpers.
- Document buffered-I/O fallback and how to check the effective mode.
- Add the option comment header and reused-device debug message.
- Remove the AI attribution trailer as requested.

Built mount, umount, losetup and findmnt with Meson. As root in a private mount namespace, loop-reuse, loop-direct-io, regfile and fstab-loop all passed. The direct-I/O test also checks boolean aliases and the conflict diagnostic.

Thanks,
Tianyi

Tianyi Chen (2):
libmount: keep reused loop devices attached on mount failure
libmount: support explicit loop direct I/O requests

 libmount/src/hook_loopdev.c         | 41 +++++++++++++-
 sys-utils/mount.8.adoc              | 13 ++++-
 tests/expected/mount/loop-direct-io |  6 +++
 tests/expected/mount/loop-reuse     |  1 +
 tests/ts/mount/loop-direct-io       | 84 +++++++++++++++++++++++++++++
 tests/ts/mount/loop-reuse           | 23 ++++++++
 6 files changed, 165 insertions(+), 3 deletions(-)
 create mode 100644 tests/expected/mount/loop-direct-io
 create mode 100644 tests/expected/mount/loop-reuse
 create mode 100755 tests/ts/mount/loop-direct-io
 create mode 100755 tests/ts/mount/loop-reuse

base-commit: 55e3c4dba6101a47ecaae71c91edbe10c59b8d56

2.55.0