libbe: Avoid double printing cloning errors.

be_clone calls be_clone_cb and both call set_error on the return
error path. set_error prints the error resulting in a double print.
be_clone_cb should just return the error code and allow be_clone
to print it.

PR: 265248
Reported by: Graham Perrin
Reviewed by: imp, kevans
Pull Request: https://github.com/freebsd/freebsd-src/pull/685

(cherry picked from commit 8e933d9c598ec847081c322e7ae9a46bf7897886)