bcachefs: Fix error in parsing of mount options

bcachefs: Fix error in parsing of mount options

When parsing the mount options duplicate the given options. This is
required as the options are parsed twice and strsep is used in parsing.
The options will be modified into a possibly invalid options set for the
second round of parsing if the options are not duplicated before
parsing.

Signed-off-by: Dan Robertson <dan@dlrobertson.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>