mmap: set 'fd' to -1

This looks like a typo, using '1' makes no sense. This fixes Solaris
anon allocs.

Reported-by: Sébastien Bouchex Bellomié <sbouchex@infovista.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
Merge branch 'master' of ssh://git.kernel.dk/data/git/fio