Make the variables rw_latch and buf_mode local in the for loop.
Initialize them at the beginning of each for loop round to reduce
register spilling on register-starved platforms such as the x86.  Move
the assignment of rw_latch and buf_mode from the end of the loop to
the beginning of the loop.  These parameters will only be needed in
the buf_page_get_gen() call at the start of the loop.

Remove the second (redundant) call to ibuf_should_try().

ibuf_should_try(): Now that the successful calls to this function will
be halved, halve the magic constant that ibuf_flush_count will be
compared to, accordingly.

rb://61 approved by Heikki over IM.