performance/write-behind:STACK_UNWIND happens only in failure path of wb_stat()
performance/write-behind:STACK_UNWIND happens only in failure path of wb_stat()

Only in case of errors STACK_UNWIND happens in wb_stat().
Also, fixed an incorrect check which would fail all writes.

Signed-off-by: Anand V. Avati <avati@dev.gluster.com>

BUG: 309 (In replicate set-up when exnihilate.sh is run , client crashes.)
URL: http://bugs.gluster.com/cgi-bin/bugzilla3/show_bug.cgi?id=309

Kaushikbv/Gluster,Kaushikbv/Gluster,Kaushikbv/Gluster