cachefiles: fix error return code in cachefiles_ondemand_copen()

The cache_size field of copen is specified by the user daemon.
If cache_size < 0, then the OPEN request is expected to fail,
while copen itself shall succeed. However, returning 0 is indeed
unexpected when cache_size is an invalid error code.

Fix this by returning error when cache_size is an invalid error code.

Changes
=======
v4: update the code suggested by Dan
v3: update the commit log suggested by Jingbo.

Fixes: c8383054506c ("cachefiles: notify the user daemon when looking up cookie")
Signed-off-by: Sun Ke <[email protected]>
Suggested-by: Jeffle Xu <[email protected]>
Suggested-by: Dan Carpenter <[email protected]>
Signed-off-by: David Howells <[email protected]>
Reviewed-by: Gao Xiang <[email protected]>
Reviewed-by: Jingbo Xu <[email protected]>
Reviewed-by: Dan Carpenter <[email protected]>
Link: https://lore.kernel.org/r/[email protected]/ # v2
Link: https://lore.kernel.org/r/[email protected]/ # v3
Link: https://lore.kernel.org/r/[email protected]/ # v4

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