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