performance/io-threads: Fix static analysis error

stub->fop can be more than FOP_MAX is what static analysis is complaining. This
patch doesn't allow any 'log' to be printed in the case fop value is not in the
definied range. It gives EINVAL instead.

Change-Id: I293381e2c1ad0ab45154b0192a637612becaf744
BUG: 1153935
Signed-off-by: Pranith Kumar K <pkarampu@redhat.com>
Reviewed-on: http://review.gluster.org/8939
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>