block: optimize I/O merge hot path with unlikely() hints

block: optimize I/O merge hot path with unlikely() hints

Remove redundant '== false' comparisons and add unlikely() branch
prediction hints in block I/O merge path functions.

These functions (ll_new_hw_segment, ll_merge_requests_fn, and
blk_rq_merge_ok) are executed on every I/O request merge attempt,
making them critical hot paths. Data integrity check failures are
rare events, so marking these conditions as unlikely() helps the
CPU optimize the common case by improving branch prediction.

Changes:
- Replace 'func() == false' with 'unlikely(!func())' for better
  code style and branch prediction

This micro-optimization reduces branch misprediction penalties in
high-frequency I/O merge paths.

Signed-off-by: Steven Feng <steven@joint-cloud.com>
Link: https://patch.msgid.link/tencent_79B652BD0CC23E093F27914380F161E7E505@qq.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>