libmount: clean nonnull attribute usage

libmount: clean nonnull attribute usage

 - use __attribute__((nonnull) for functions where we not able to
   return an return code ("is", "has" and some "get" functions).

 - use __attribute__((nonnull) for small functions where we
   always modify any of the function argument (some mnt_optstr_* functions)

Signed-off-by: Karel Zak <kzak@redhat.com>