This introduces a new error code, option_needs_open_fs, which is used to
indicate that an attempt was made to parse a mount option prior to
opening a filesystem, when that mount option requires an open filesystem
in order to be validated.

Returning this error results in bch2_parse_one_mount_opt() saving that
option for later parsing, after the filesystem is opened.
Signed-off-by: Thomas Bertschinger <tahbertschinger@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>