bcachefs: early return for negative values when parsing BCH_OPT_UINT

bcachefs: early return for negative values when parsing BCH_OPT_UINT

Currently, when passing a negative integer as argument, the error
message is "too big" due to casting to an unsigned integer:

    > bcachefs format --block_size=-1 bcachefs.img
    invalid option: block_size: too big (max 65536)

When negative value in argument detected, return early before
calling bch2_opt_validate().

A new error code `BCH_ERR_option_negative` is added.

Signed-off-by: Integral <integral@archlinuxcn.org>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>