dm crypt: fix get_key_size compiler warning if !CONFIG_KEYS

[ Upstream commit 6fc51504388c1a1a53db8faafe9fff78fccc7c87 ]

Explicitly convert unsigned int in the right of the conditional
expression to int to match the left side operand and the return type,
fixing the following compiler warning:

drivers/md/dm-crypt.c:2593:43: warning: signed and unsigned
type in conditional expression [-Wsign-compare]

Fixes: c538f6ec ("dm crypt: add ability to use keys from the kernel key retention service")
Signed-off-by: Aashish Sharma <shraash@google.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>