Use vmem_alloc() for zfs_ioc_userspace_many()

Use vmem_alloc() for zfs_ioc_userspace_many()

The default buffer size when requesting multiple quota entries
is 100 times the zfs_useracct_t size.  In practice this works out
to exactly 27200 bytes.  Since this will be a short lived buffer
in a non-performance critical path it is preferable to vmem_alloc()
the needed memory.