LU-6142 lustre: iput() can safely be passed NULL.

LU-6142 lustre: iput() can safely be passed NULL.

iput() is a no-op when passed a NULL pointer, so there is no
need to test for NULL before calling it - doing so clutters
the code.

Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: Idcd1a6746ecc67dcfcb0713d2762ca0bdb29de19
Reviewed-on: https://review.whamcloud.com/40291
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Aurelien Degremont <degremoa@amazon.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>