When an active g_vfs is orphaned due to an underlying disk going away
the destroy is deferred until the filesystem is unmounted in
g_vfs_done(). However, g_vfs_done() is invoked from a non-sleepable
context and cannot use M_WAITOK to allocate the event. Instead,
allocate the event in g_vfs_orphan() and save it in the softc to be
retrieved by the last call to g_vfs_done().

Reported by: Jithesh Arakkan @ Chelsio
Sponsored by: Chelsio Communications