Fix fsck_ffs -R finds unfixed duplicate block errors when rerunning.

This fixes a long-standing but very obscure bug in fsck_ffs when
it is run with the -R (rerun after unexpected errors). It only
occurs if fsck_ffs finds duplicate blocks and they are all contained
in inodes that reside in the first block of inodes (typically among
the first 128 inodes).

Rather than use the usual ginode() interface to walk through the
inodes in pass1, there is a special optimized `getnextinode()'
routine for walking through all the inodes. It has its own private
buffer for reading the inode blocks. If pass 1 finds duplicate
blocks it runs pass 1b to find all the inodes that contain these
duplicate blocks. Pass 1b also uses the `getnextinode()' to search
for the inodes with duplicate blocks. Pass 1b stops when all the
duplicate blocks have been found. If all the duplicate blocks are
found in the first block of inodes, then the getnextinode cache
holds this block of bad inodes. The subsequent cleanup of the inodes
in passes 2-5 is done using ginode() which uses the regular fsck_ffs
cache.

When fsck_ffs restarts, pass1() calls setinodebuf() to point at the
first block of inodes. When it calls getnextinode() to get inode
2, getnextino() sees that its private cache already has the first
set of inodes loaded and starts using them. They are of course the
trashed inodes left over from the previous run of pass1b().

The fix is to always invalidate the getnextinode cache when calling
setinodebuf().

Reported by: Chuck Silvers
Tested by: Chuck Silvers
MFC after: 3 days
Sponsored by: Netflix