Rename ffz() function to fio_ffz() to avoid conflicts

Newer gcc/glibc include an ffz function. Lets just do the easy
thing and rename ours to fio_ffz(), then we can look into reusing
a provided one later if necessary.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/io_u.c b/io_u.c
index 7fe5242..e892ec7 100644
--- a/io_u.c
+++ b/io_u.c
@@ -84,7 +84,7 @@
 	*b = (i * BLOCKS_PER_MAP);
 	while ((*b) * td->o.rw_min_bs < f->real_file_size) {
 		if (f->file_map[i] != -1UL) {
-			*b += ffz(f->file_map[i]);
+			*b += fio_ffz(f->file_map[i]);
 			f->last_free_lookup = i;
 			return 0;
 		}