MFC r268971 and r269032.

MFC r268971 and r269032.

r268971:
Simplify r267600, there's no need to distinguish between allocated and
inlined mbufs.

r269032:
cxgbe(4):  Keep track of the clusters that have to be freed by the
custom free routine (rxb_free) in the driver.  Fail MOD_UNLOAD with
EBUSY if any such cluster has been handed up to the kernel but hasn't
been freed yet.  This prevents a panic later when the cluster finally
needs to be freed but rxb_free is gone from the kernel.

git-svn-id: svn://svn.freebsd.org/base/stable/10@269356 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f