Backed out previous commit. It cause massive filesystem corruption,

not to mention a compile-time warning about the critical function
becoming unused, by replacing spec_bmap() with vop_stdbmap().

ntfs seems to have the same bug.

The factor for converting specfs block numbers to physical block
numbers is 1, but vop_stdbmap() uses the bogus factor
btodb(ap->a_vp->v_mount->mnt_stat.f_iosize), which is 16 for ffs with
the default block size of 8K.  This factor is bogus even for vop_stdbmap()
-- the correct factor is related to the filesystem blocksize which is not
necessarily the same to the optimal i/o size.  vop_stdbmap() was apparently
cloned from nfs where these sizes happen to be the same.

There may also be a problem with a_vp->v_mount being null.  spec_bmap()
still checks for this, but I think the checks in specfs are dead code
which used to support block devices.

sys/fs/specfs/spec_vnops.c

@@ -73,6 +73,7 @@ static struct vnodeopv_entry_desc spec_vnodeop_entries[] = {
 	{ &vop_default_desc,		(vop_t *) vop_defaultop },
 	{ &vop_access_desc,		(vop_t *) vop_ebadf },
 	{ &vop_advlock_desc,		(vop_t *) spec_advlock },
+	{ &vop_bmap_desc,		(vop_t *) spec_bmap },
 	{ &vop_close_desc,		(vop_t *) spec_close },
 	{ &vop_create_desc,		(vop_t *) vop_panic },
 	{ &vop_freeblks_desc,		(vop_t *) spec_freeblks },

sys/miscfs/specfs/spec_vnops.c

@@ -73,6 +73,7 @@ static struct vnodeopv_entry_desc spec_vnodeop_entries[] = {
 	{ &vop_default_desc,		(vop_t *) vop_defaultop },
 	{ &vop_access_desc,		(vop_t *) vop_ebadf },
 	{ &vop_advlock_desc,		(vop_t *) spec_advlock },
+	{ &vop_bmap_desc,		(vop_t *) spec_bmap },
 	{ &vop_close_desc,		(vop_t *) spec_close },
 	{ &vop_create_desc,		(vop_t *) vop_panic },
 	{ &vop_freeblks_desc,		(vop_t *) spec_freeblks },