Use the reentrant getmntent_r instead of getmntent

Use the reentrant getmntent_r instead of getmntent

The function find_cgroup_mnt() could be called under multithread scenario, so
we should use the reentrant edition getmntent_r() instead of getmntent().

E.g, without this fix the output for below job file is wrong.

[root@localhost blkio-test]# cat small-read-file
[global]
ioengine=sync
direct=1
rw=read
bs=4k
filename=/dev/sdb
time_based
runtime=60
cgroup=small
cgroup_weight=500
numjobs=32
group_reporting
thread

[file1]

Then fio randomly says:
[cut here]
Starting 32 threads
fio: cgroup blkio does not appear to be mounted
fio: cgroup blkio does not appear to be mounted
fio: cgroup blkio does not appear to be mounted

Signed-off-by: Jens Axboe <axboe@kernel.dk>