variable name is misleading,please fix it

[shanghaifei]

in the module store,in the file org/apache/rocketmq/store/MappedFile.java
in the method isAbleToCommit()
please use the variable name 'commit' instead variable name 'flush'
it's wrong,please fix it

protected boolean isAbleToCommit(final int commitLeastPages) {
        // this is commit position，not flush position
        // int flush = this.committedPosition.get();
        int commit = this.committedPosition.get();
        int write = this.wrotePosition.get();

        if (this.isFull()) {
            return true;
        }

        if (commitLeastPages > 0) {
            //return ((write / OS_PAGE_SIZE) - (flush / OS_PAGE_SIZE)) >= commitLeastPages;
            return ((write / OS_PAGE_SIZE) - (commit / OS_PAGE_SIZE)) >= commitLeastPages;
        }

        //return write > flush;
        return write > commit;
    }