HAMMER VFS - Better CRC handling, bad-file handling.

HAMMER VFS - Better CRC handling, bad-file handling.

Data CRC errors should now generate EIO instead of panic()ing the system.
B-Tree CRC errors might still panic() and freemap CRC errors WILL still
panic().

Continuing from DDB on a B-Tree node CRC error when debugging is enabled
now no longer marks the B-Tree node as good.

The mirror-read command will now transfer data records with bad CRCs
instead of aborting the transfer, identifying them with a new type field.
The mirror-write ioctl currently ignores such records.

If a directory entry is encountered and the related inode cannot be
looked up, generate a dummy in-memory inode of type FIFO to placemark
the bad directory entry, allowing it to be removed.  Currently it is
possible for a directory entry to be synced to the media in a different
transaction then the related inode (a bug which needs to be fixed).
If a crash occurs at the wrong time the recovery code can leave the media
in a state where the directory entry exists but the inode does not.  This
change allows the bad directory entry to be removed.

Reported-by: Antonio Huete Jimenez