move skip_bad= option to engines/mtd.c

move skip_bad= option to engines/mtd.c

This is ioengine specific, thus should be defined in engines/mtd.c.

This option is useless unless engines/mtd.c is enabled.
It could be conditionally compiled using CONFIG_MTD, but all
other ioengines define their own option structure in their code.

The only functional difference is that now .category is changed
to FIO_OPT_C_ENGINE. This is also something done by all other
ioengine specific options, though ->category isn't really used
for anything meaningfull.

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