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log_t::resize_write(): Advance log_sys.resize_lsn and reset
the resize_log offset to START_OFFSET whenever the memory-mapped buffer
would wrap around.

Previously, in case the initial target offset would be beyond the
requested innodb_log_file_size, we only adjusted the offset but
not the LSN. An incorrect LSN would cause log_sys.buf_free to be out
of bounds when the log resizing completes.

The log_sys.lsn_lock will cover the entire duration of replicating
memory-mapped log for resizing. We just need a mutex that is compatible
with the caller holding log_sys.latch. While the choice of mtr_t::finisher
(for normal log writes) depends on mtr_t::spin_wait_delay,
replicating the log during resizing is a rare operation where we can
afford possible additional context switching overhead.