vfs_default: Fix passing of errno from async calls

vfs_default: Fix passing of errno from async calls

Current code assigns errno from async pthreadpool calls to the
vfs_default internal vfswrap_*_state.  The callers of the vfs_*_recv
functions expect the value from errno in vfs_aio_state.error.

Correctly assign errno to vfs_aio_state.error and remove the unused
internal err variable.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=12983

Signed-off-by: Christof Schmitt <cs@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>