Add new kstat for monitoring time in dmu_tx_assign

This change adds a new kstat to gain some visibility into the amount of
time spent in each call to dmu_tx_assign. A histogram is exported via
a new dmu_tx_assign_histogram-$POOLNAME file. The information contained
in this histogram is the frequency dmu_tx_assign took to complete given
an interval range. For example, given the below histogram file:

    $ cat /proc/spl/kstat/zfs/dmu_tx_assign_histogram-tank
    12 1 0x01 32 1536 19792068076691 20516481514522
    name                            type data
    1 us                            4    859
    2 us                            4    252
    4 us                            4    171
    8 us                            4    2
    16 us                           4    0
    32 us                           4    2
    64 us                           4    0
    128 us                          4    0
    256 us                          4    0
    512 us                          4    0
    1024 us                         4    0
    2048 us                         4    0
    4096 us                         4    0
    8192 us                         4    0
    16384 us                        4    0
    32768 us                        4    1
    65536 us                        4    1
    131072 us                       4    1
    262144 us                       4    4
    524288 us                       4    0
    1048576 us                      4    0
    2097152 us                      4    0
    4194304 us                      4    0
    8388608 us                      4    0
    16777216 us                     4    0
    33554432 us                     4    0
    67108864 us                     4    0
    134217728 us                    4    0
    268435456 us                    4    0
    536870912 us                    4    0
    1073741824 us                   4    0
    2147483648 us                   4    0

one can see most calls to dmu_tx_assign completed in 32us or less, but a
few outliers did not. Specifically, 4 of the calls took between 262144us
and 131072us. This information is difficult, if not impossible, to gather
without this change.

Signed-off-by: Prakash Surya <surya1@llnl.gov>