bcachefs: fix the error code when mounting with incorrect options.

When mount with incorrect options such as:
"mount -t bcachefs -o errors=back /dev/loop1 /mnt/bcachefs/".
It rebacks the error "mount: /mnt/bcachefs: permission denied."
 cause bch2_parse_mount_opts returns -1 and bch2_mount throws
it up. This is unreasonable.

The real error message should be like this:
"mount: /mnt/bcachefs: wrong fs type, bad option, bad
superblock on /dev/loop1, missing codepage or helper program,
or other error."

Adding three private error codes for mounting error. Here are:
  - BCH_ERR_mount_option as the parent class for option error.
  - BCH_ERR_option_name represents the invalid option name.
  - BCH_ERR_option_value represents the invalid option value.

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