Clear PG_writeback for sync I/O error case

Commit 2b2861362f7dd09cc3167df8fddb6e2cb475018a accidentally
introduced this issue by only conditionally registering the
commit callback in the async case.

The error handing code for the dmu_tx_assign() failure case
relied on there always being a registered commit callback to
clear the PG_writeback bit.  Since that is no longer strictly
true for the synchronous case we must explicitly invoke the
callback.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #961
ZFS replay transaction error 5

When zfs_replay_write() replays TX_WRITE records from ZIL
it calls zpl_write_common() to perform the actual write.
zpl_write_common() returns the number of bytes written
(similar to write() system call) or an (negative) error.
However, the code expects the positive return value to be
a residual counter. Thus when zpl_write_common() successfully
completes it is mistakenly considered to be a partial write and
the error code delivered further. At this point the ZIL processing
is aborted with famous "ZFS replay transaction error 5" error
message given to the message buffer.

The fix is to compare the zpl_write_commmon() return value with
the buffer size and flag error only when they disagree.

Signed-off-by: Cyril Plisko <cyril.plisko@mountall.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #933
Switch KM_SLEEP to KM_PUSHPAGE

This warning indicates the incorrect use of KM_SLEEP in a call
path which must use KM_PUSHPAGE to avoid deadlocking in direct
reclaim.  See commit b8d06fca089fae4680c3a552fc55c512bfb02202
for additional details.

  SPL: Fixing allocation for task txg_sync (6093) which
  used GFP flags 0x297bda7c with PF_NOFS set

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #917
Improve `zpool import` search behavior

The goal of this change is to make 'zpool import' prefer to use
the peristent /dev/mapper or /dev/disk/by-* paths.  These are far
preferable to the devices in /dev/ whos names are not persistent
and are determined by the order in which a device is detected.

This patch improves things by changing the default search path from
just to the top level /dev/ directory to (in order):

  /dev/disk/by-vdev   - Custom rules, use first if they exist
  /dev/disk/zpool     - Custom rules, use first if they exist
  /dev/mapper         - Use multipath devices before components
  /dev/disk/by-uuid   - Single unique entry and persistent
  /dev/disk/by-id     - May be multiple entries and persistent
  /dev/disk/by-path   - Encodes physical location and persistent
  /dev/disk/by-label  - Custom persistent labels
  /dev                - UNSAFE device names will change

The default search path can be overriden by setting the
ZPOOL_IMPORT_PATH environment variable.  This must be a colon
delimited list of paths which are searched for vdevs.  If the
'zpool import -d' option is specified only those listed paths
will be searched.

Finally, when multiple paths to the same device are found.  If one
of the paths is an exact match for the path used last time to import
the pool it will be used.  When there are no exact matches the
prefered path will be determined by the provided search order.

This means you can still import a pool and force specific names by
providing the -d <path> option.  And the prefered names will persist
as long as those paths exist on your system.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #965