LU-15404 ldiskfs: truncate during setxattr leads to kernel panic

LU-15404 ldiskfs: truncate during setxattr leads to kernel panic

When changing a large xattr value to a different large xattr value,
the old xattr inode is freed. Truncate during the final iput causes
current transaction restart. Eventually, parent inode bh is marked
dirty and kernel panic happens when jbd2 figures out that this bh
belongs to the committed transaction.

A possible fix is to call this final iput in a separate thread.
This way, setxattr transactions will never be split into two.
Since the setxattr code adds xattr inodes with nlink=0 into the
orphan list, old xattr inodes will be properly cleaned up in
any case.

Change-Id: Idd70befa6a83818ece06daccf9bb6256812674b9
Signed-off-by: Andrew Perepechko <andrew.perepechko@hpe.com>
HPE-bug-id: LUS-10534
Reviewed-on: https://review.whamcloud.com/46358
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Alexander Zarochentsev <alexander.zarochentsev@hpe.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>