libmount: check for errors from prep-options hooks

libmount: check for errors from prep-options hooks

fix_optstr() in context_mount.c calls the MNT_STAGE_PREP_OPTIONS hooks
but discards returned errors. This suppresses error messages and can
make invalid option strings 'fail open' in surprising ways. Handle error
returns from these hooks correctly.

No other callers of mnt_context_call_hooks() discard the return value,
so other hooks aren't affected.

Signed-off-by: Chris Webb <chris@arachsys.com>