LU-6142 lustre: use is_root_inode()

LU-6142 lustre: use is_root_inode()

Lustre has multiple tests to see if a given inode is the root of the
filesystem.  Linux has (since 3.19) a helper function is_root_inode().
Use that throughout.

Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: Ib5997e8976b6e935c268ad7ac9328fcd4d071b1f
Reviewed-on: https://review.whamcloud.com/40293
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>