LU-14459 llite: reset pfid after dir migration

LU-14459 llite: reset pfid after dir migration

A plain directory will be turned into to a stripe upon
migration/restripe, and reversely if target is plain directory, the
target stripe will be turned into directory after.

In the first case, set pfid, and in the latter case, clear pfid,
otherwise ll_lock_cancel_bits() will use the wrong master inode.

Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: I01cac0103dc79d493166e6b090508d24f9678a57
Reviewed-on: https://review.whamcloud.com/43289
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Hongchao Zhang <hongchao@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>