f2fs: support fault_type mount option

Previously, once fault injection is on, by default, all kind of faults
will be injected to f2fs, if we want to trigger single or specified
combined type during the test, we need to configure sysfs entry, it will
be a little inconvenient to integrate sysfs configuring into testsuit,
such as xfstest.

So this patch introduces a new mount option 'fault_type' to assist old
option 'fault_injection', with these two mount options, we can specify
any fault rate/type at mount-time.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>