fs_parse: allow parameter value to be empty

Allow parameter value to be empty by specifying fs_param_can_be_empty
flag.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Link: https://lore.kernel.org/r/20211027141857.33657-2-lczerner@redhat.com


Signed-off-by: Theodore Ts'o <tytso@mit.edu>