seq_buf: Clear what a writer did not claim when a seq_buf overflows

Using seq_buf_set_overflow() would leave the bytes between "len"
and "size" untouched, so if seq_buf_str() is used on an overflowed
seq_buf, those bytes may be exposed. For any paths that don't claim
partially written bytes, by setting "len = size" before calling
seq_buf_set_overflow(), wipe the unclaimed bytes. The seq_buf_puts()
and related APIs already claim those bytes now, so only the unclaimed
cases remain. A specific example of this was seq_buf_path() which uses
d_path() and would write to the tail before discovering it was out
of space, and would correctly mark a seq_buf as overflowed, but the
path fragment would be left over.

Clear from len to the end of the buffer in seq_buf_set_overflow(), which
every overflow goes through, including seq_buf_commit() with a negative
count.

Add a test that fills a seq_buf, leaves it too little room for a path, and
checks that nothing of the path is left in the buffer. The tests run before
anything writable is mounted, so it takes its file from shmem.

seq_buf_path() was not exported, so the test failed to link as a
module. Export it.

Tests passed under qemu on ARCH=x86_64 with GCC 16.2.0 and CONFIG_KASAN=y,
and on big-endian ARCH=s390 with GCC s390x-linux-gnu 16.2.0, and the
test builds as a module (CONFIG_SEQ_BUF_KUNIT_TEST=m).

Assisted-by: LLM
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://patch.msgid.link/20261005155708.1471260-5-kees@kernel.org
Signed-off-by: Kees Cook <kees@kernel.org>

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