bcachefs: intercept mountoption value for bool type

For mount option with bool type, the value must be 0 or 1 (See
bch2_opt_parse). But this seems does not well intercepted cause
for other value(like 2...), it returns the unexpect return code
with error message printed.

Signed-off-by: Hongbo Li <lihongbo22@huawei.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>