fs: don't let statmount return empty strings

When one of the statmount_string() handlers doesn't emit anything to
seq, the kernel currently sets the corresponding flag and emits an empty
string.

Given that statmount() returns a mask of accessible fields, just leave
the bit unset in this case, and skip any NULL termination. If nothing
was emitted to the seq, then the EOVERFLOW and EAGAIN cases aren't
applicable and the function can just return immediately.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://lore.kernel.org/r/20241111-statmount-v4-1-2eaf35d07a80@kernel.org


Acked-by: Miklos Szeredi <mszeredi@redhat.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>