libgomp: Release device lock on cbuf error path

This patch releases the device lock on a sanity-checking error path in
transfer combining (cbuf) handling in libgomp:target.c.  This shouldn't
happen when handling well-formed mapping clauses, but erroneous clauses
can currently cause a hang if the condition triggers.

2021-12-10  Julian Brown  <julian@codesourcery.com>

libgomp/
	* target.c (gomp_copy_host2dev): Release device lock on cbuf
	error path.