crash dump support: incorrect argument on commit
lib_ring_buffer_write_commit_counter()'s 'buf_offset' argument should
contain offset of beginning of area used by the record being comitted.
However, lib_ring_buffer_commit() passes ctx->buf_offset, that gets
advanced by lib_ring_buffer_write() and thus points to just-after-
end-of-record at lib_ring_buffer_commit() time. This causes
lib_ring_buffer_write_commit_counter() to return without changing
commit_hot[idx].seq, due to
if (unlikely(subbuf_offset(offset - commit_count, chan)))
Since after-crash data extraction tool checks 'seq' field to find out
how much data is in buffer, this results into inavailability of
data from partially-filled subbuffer for after-crash analysis.