dm-pcache: use int type to store negative error codes

Change the 'ret' variable from u32 to int to store negative error codes or
zero returned by cache_kset_close().

Storing the negative error codes in unsigned type, doesn't cause an issue
at runtime but it's ugly. Additionally, assigning negative error codes to
unsigned type may trigger a GCC warning when the -Wsign-conversion flag
is enabled.

No effect on runtime.

Signed-off-by: Qianfeng Rong <rongqianfeng@vivo.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>