afs: Fix an IS_ERR() vs NULL check

afs: Fix an IS_ERR() vs NULL check

[ Upstream commit a33d62662d275cee22888fa7760fe09d5b9cd1f9 ]

The proc_symlink() function returns NULL on error, it doesn't return
error pointers.

Fixes: 5b86d4ff5dce ("afs: Implement network namespacing")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: linux-afs@lists.infradead.org
Link: https://lore.kernel.org/r/YLjMRKX40pTrJvgf@mwanda/
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>