Large sync writes perform worse with slog

For synchronous write workloads with large IO sizes, a pool configured
with a slog performs worse than one with an embedded zil:

sequential_writes 1m sync ios, 16 threads

  Write IOPS:              1292          438   -66.10%
  Write Bandwidth:      1323570       448910   -66.08%
  Write Latency:       12128400     36330970      3.0x

sequential_writes 1m sync ios, 32 threads

  Write IOPS:              1293          430   -66.74%
  Write Bandwidth:      1324184       441188   -66.68%
  Write Latency:       24486278     74028536      3.0x

The reason is the zil_slog_bulk variable. In zil_lwb_write_open,
if a zil block is greater than 768K, the priority of the write is
downgraded from sync to async. Increasing the value allows greater
throughput. To select a value for this PR, I ran an fio workload with
the following values for zil_slog_bulk:

    zil_slog_bulk    KiB/s
    1048576         422132
    2097152         478935
    4194304         533645
    8388608         623031
    12582912        827158
    16777216       1038359
    25165824       1142210
    33554432       1211472
    50331648       1292847
    67108864       1308506
    100663296      1306821
    134217728      1304998

At 64M, the results with a slog are now improved to parity with an
embedded zil:

sequential_writes 1m sync ios, 16 threads

  Write IOPS:               438         1288      2.9x
  Write Bandwidth:       448910      1319062      2.9x
  Write Latency:       36330970     12163408   -66.52%

sequential_writes 1m sync ios, 32 threads

  Write IOPS:               430         1290      3.0x
  Write Bandwidth:       441188      1321693      3.0x
  Write Latency:       74028536     24519698   -66.88%

None of the other tests in the performance suite (run with a zil or
slog) had a significant change, including the random_write_zil tests,
which use multiple datasets.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com>
Signed-off-by: John Wren Kennedy <john.kennedy@delphix.com>
Closes #14378