fs: crypto: Maintain reference count for class keys

Reference count for device queue might overflow in case of class
key if we keep preparing keys for each inode. So maintain a ref count
to ensure keys can be fetched for inodes belonging to same storage class.

Test: Tested by customer and it fixes the issue.
Change-Id: Ia1dd32d0e0feeb79aa40335bd32b0ab6b693b440
Signed-off-by: Neeraj Soni <quic_neersoni@quicinc.com>