HOWTO: Add note/exception on allow_mounted_write=

HOWTO: Add note/exception on allow_mounted_write=

allow_mounted_write= is useful on Linux, but some platforms
just return some error on open(2) for mounted devices.

 # uname
 Linux
 # mkfs.ext4 /dev/loop0 > /dev/null
 mke2fs 1.43.3 (04-Sep-2016)
 # mount /dev/loop0 /mnt
 # mount | grep /dev/loop0
 /dev/loop0 on /mnt type ext4 (rw,relatime,seclabel,data=ordered)
 # fio --name=xxxxx --ioengine=sync --rw=write --bs=32k --size=1m --filename=/dev/loop0
 xxxxx: (g=0): rw=write, bs=32.0KiB-32.0KiB,32.0KiB-32.0KiB,32.0KiB-32.0KiB, ioengine=sync, iodepth=1
 fio-2.18-21-gca205
 fio: /dev/loop0 appears mounted, and 'allow_mounted_write' isn't set. Aborting.
 Run status group 0 (all jobs):
 # fio --name=xxxxx --ioengine=sync --rw=write --bs=32k --size=1m --filename=/dev/loop0 --allow_mounted_write=1 > /dev/null; echo $?
 0

 # uname
 FreeBSD
 # newfs /dev/da1 > /dev/null
 # mount /dev/da1 /mnt
 # mount | grep /dev/da1
 /dev/da1 on /mnt (ufs, local)
 # fio --name=xxxxx --ioengine=sync --rw=write --bs=32k --size=1m --filename=/dev/da1 --allow_mounted_write=1
 fio: this platform does not support process shared mutexes, forcing use of threads. Use the 'thread' option to get rid of this warning.
 xxxxx: (g=0): rw=write, bs=32.0KiB-32.0KiB,32.0KiB-32.0KiB,32.0KiB-32.0KiB, ioengine=sync, iodepth=1
 fio-2.18-21-gca205
 Starting 1 thread
 fio: failed opening chardev /dev/da1 for size check
 file:filesetup.c:654, func=open(/dev/da1), error=Operation not permitted
 fio: pid=0, err=1/file:filesetup.c:654, func=open(/dev/da1), error=Operation not permitted
 Run status group 0 (all jobs)

Signed-off-by: Tomohiro Kusumi <tkusumi@tuxera.com>
Signed-off-by: Jens Axboe <axboe@fb.com>