HAMMER VFS - Handle critical I/O errors without panicing

HAMMER VFS - Handle critical I/O errors without panicing

Physically pull the SATA cable while pounding the disk via a HAMMER
mounted filesystem and fix the error paths that are not handled properly.
Make umount -f work properly.

Reminder: Currently if HAMMER hits a critical media error it drops down
into read-only mode and can only be read or unmounted after that.

* Do not try to test the CRC for bread data if the bread fails.

* Clean up the reference count on hammer_buffer structures related to
  dangling modified and ioerror bits.

* Remove a debugging kprintf() that was lousing up the console output.