Increase ZFS_OBJ_MTX_SZ to 256

Increase ZFS_OBJ_MTX_SZ to 256

Increasing this limit costs us 6144 bytes of memory per mounted
filesystem, but this is small price to pay for accomplishing
the following:

* Allows for up to 256-way concurreny when performing lookups
  which helps performance when there are a large number of
  processes.

* Minimizes the likelyhood of encountering the deadlock
  described in issue #1101.  Because vmalloc() won't strictly
  honor __GFP_FS there is still a very remote chance of a
  deadlock.  See the zfsonlinux/spl@043f9b57 commit.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1101