libnvdimm, btt: fix uninitialized err_lock

When a sector mode namespace is initially created, the arena's err_lock
is not initialized.  If, on the other hand, the namespace already
exists, the mutex is initialized.  To fix the issue, I moved the mutex
initialization into the arena_alloc, which is called by both
discover_arenas and create_arenas.

This was discovered on an older kernel where mutex_trylock checks the
count to determine whether the lock is held.  Because the data structure
is kzalloc-d, that count was 0 (held), and I/O to the device would hang
forever waiting for the lock to be released (see btt_write_pg, for
example).  Current kernels have a different mutex implementation that
checks for a non-null owner, and so this doesn't show up as a problem.
If that lock were ever contended, it might cause issues, but you'd have
to be really unlucky, I think.

Signed-off-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>