mount.cifs: have parse_options fill parsed_mount_info

mount.cifs: have parse_options fill parsed_mount_info

Allocate a zeroed out parsed_mount_info struct and have parse_options
put its info into that instead. realloc() is no longer used here and
instead we just have the option parser carefully check that the result
will fit in the buffer before copying it.

We also no longer use snprintf to stuff info directly into the buffer.
It may not be possible given the other checks, but snprintf can leave a
non-NULL terminated string. Use strlcat everywhere instead to ensure
that doesn't occur.

Signed-off-by: Jeff Layton <jlayton@redhat.com>