block/rnbd: fix a null pointer dereference on dev->blk_symlink_name

block/rnbd: fix a null pointer dereference on dev->blk_symlink_name

Currently in the case where dev->blk_symlink_name fails to be allocates
the error return path attempts to set an end-of-string character to
the unallocated dev->blk_symlink_name causing a null pointer dereference
error. Fix this by returning with an explicity ENOMEM error (which also
is missing in the original code as was not initialized).

Fixes: 1eb54f8f5dd8 ("block/rnbd: client: sysfs interface functions")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Addresses-Coverity: ("Dereference after null check")
Signed-off-by: Jens Axboe <axboe@kernel.dk>