jffs2: hold erase_completion_lock on exit

Users of jffs2_do_reserve_space() expect they still held
erase_completion_lock after call to it. But there is a path
where jffs2_do_reserve_space() leaves erase_completion_lock unlocked.
The patch fixes it.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Cc: stable@vger.kernel.org
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>