handle disk failures

handle disk failures

From: Dan Williams <dan.j.williams@intel.com>

Added curr_state as a parameter to set_disk.  Handlers look at this to
record components failures, and set global 'degraded' or 'failed'
status.

When reading the state as faulty:
1/ mark the disk failed in the metadata

2/ write '-blocked' to the rdev state to allow the kernel's failure
   mechanism to advance

3/ the kernel will take away the drive's role in remove_and_add_spares()

4/ once the disk no longer has a role writing 'remove' to the rdev state
   will get the disk out of array.

There is a window after writing '-blocked' where the kernel will return
-EBUSY to remove requests.  We rely on the fact that the disk will
continue to show faulty so we lazily wait until the kernel is ready to
remove the disk.  If the manager thread needs to get the disk out of the
way it can ping the monitor and wait, just like the replace_array()
case.

[buglet fix: swap the parameters of attr_match in read_dev_state]

Signed-off-by: Dan Williams <dan.j.williams@intel.com>