tail: avoid unnecessary pipe2 syscall for empty files

[dragutreis]

Fixes #12464

bounded_tail calls print_target_section unconditionally, which creates
a broker pipe via splice_unbounded_broker even when the file is empty or
exhausted, causing an unnecessary pipe2 syscall.

Added is_file_exhausted to check before calling print_target_section:

• Regular files: compare stream position against file length
• Char/block devices: probe with a single-byte read; if data exists, write
  it directly to stdout since char devices don't support seeking back

Before:
strace -c -e pipe,pipe2 tail -c +1 /dev/null → 1 pipe2 call
After:
strace -c -e pipe,pipe2 tail -c +1 /dev/null → 0 pipe2 calls

[oech3]

Does this require additional stat or other syscall? I'm not sure which syscall is cheap, but I thins this is not needed considering usage for 0-sized file is very rare.

[dragutreis]

Compared full syscall counts with strace -c:

Empty file (/dev/null):

• Without fix: 129 syscalls (includes pipe2 + splice)
• With fix: 124 syscalls

No new syscalls added. statx goes down by 1.

Non-empty file:

• Without fix: 124 syscalls
• With fix: 124 syscalls

No overhead for the common case.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/pr/bounded-memory (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/tail-n0f (was skipped on 'main', now failing)

[oech3]

it directly to stdout since char devices don't support seeking back

Are /dev/sd* and /dev/nvme* missing splice driver yet? I saw cat does not call splice yet. (Though I want to cover the case kernel started supporting someday).

[dragutreis]

it directly to stdout since char devices don't support seeking back

Are /dev/sd* and /dev/nvme* missing splice driver yet? I saw cat does not call splice yet. (Though I want to cover the case kernel started supporting someday).

tested on /dev/nvme0n1, splice is called successfully, so block devices do support it

[sylvestre]

please remove this allow

[dragutreis]

alright