tmpfs: remove bogus MPASS(VOP_ISLOCKED(vp)) asserts

[kib]

Summary

VOP_ISLOCKED() does not return bool, its only reliable use it to check
that the vnode is exclusively locked by the calling thread.  Almost all
asserts of this form repeated auto-generated assertions from
vnode_if.src for VOPs.

In two places where the assertions would be meaningful, convert them to
ASSERT_VOP_LOCKED() statements.

[mjg]

that IFF macro should probably get gc'ed

it happens to have other uses right now though:

fs/tmpfs/tmpfs_subr.c:  MPASS(IFF(type == VLNK, target != NULL));
fs/tmpfs/tmpfs_subr.c:  MPASS(IFF(type == VBLK || type == VCHR, rdev != VNOVAL));

[kib]

tmpfs: remove IFF macro

[markj]

sys/fs/tmpfs/tmpfs_vnops.c

228

The old version also asserted MPASS(error == 0 || *vpp == NULL).