Clean up and document UFS/FFS error returns.

The ffs_inotovp() function returns a vnode from a mounted filesystem
for an inode number with specified generation number. We now
consistently return ESTALE if the inode with given generation number
no longer exists on that filesystem.

The ffs_reload() function reloads all incore data for a filesystem.
It is used after running fsck on a mounted filesystem and finding
things to fix. It now returns the EINTEGRITY error if it is unable
to find a valid superblock.

MFC-after: 1 week
Sponsored-by: The FreeBSD Foundation