Staging: lustre: llite: Simplify error handling

This patch fixes the following checkpatch error:
ERROR: do not use assignment in if condition
2172: FILE: drivers/staging/lustre/lustre/llite/llite_lib.c:2172:
        if (!inode || !(sbi = ll_i2sbi(inode))) {

Signed-off-by: Tina Ruchandani <ruchandani.tina@gmail.com>
Suggested-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>