bcache: avoid NULL checking to c->root in run_cache_set()

[ Upstream commit 3eba5e0b

 ]

In run_cache_set() after c->root returned from bch_btree_node_get(), it
is checked by IS_ERR_OR_NULL(). Indeed it is unncessary to check NULL
because bch_btree_node_get() will not return NULL pointer to caller.

This patch replaces IS_ERR_OR_NULL() by IS_ERR() for the above reason.

Signed-off-by: Coly Li <colyli@suse.de>
Link: https://lore.kernel.org/r/20231120052503.6122-11-colyli@suse.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>