xfs: unmapped buffer item size straddling mismatch

xfs: unmapped buffer item size straddling mismatch

We never log large contiguous regions of unmapped buffers, so this
bug is never triggered by the current code. However, the slowpath
for formatting buffer straddling regions is broken.

That is, the size and shape of the log vector calculated across a
straddle does not match how the formatting code formats a straddle.
This results in a log vector with an uninitialised iovec and this
causes a crash when xlog_write_full() goes to copy the iovec into
the journal.

Whilst touching this code, don't bother checking mapped or single
folio buffers for discontiguous regions because they don't have
them. This significantly reduces the overhead of this check when
logging large buffers as calling xfs_buf_offset() is not free and
it occurs a *lot* in those cases.

Fixes: 929f8b0deb83 ("xfs: optimise xfs_buf_item_size/format for contiguous regions")
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>