Fix invalid context bug

In the module unload path the vm_file_cache was being destroyed
under a spin lock. Because this operation might sleep it was
possible, although very very unlikely, that this could result
in a deadlock.

This issue was indentified by using a Linux debug kernel and
has been fixed by moving the kmem_cache_destroy() out from under
the spin lock. There is no need to lock this operation here.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes zfsonlinux/zfs#771