include/linux/cleancache.h: Clean up code

Let cleancache_fs_enabled() call cleancache_fs_enabled_mapping()
directly.

Remove redundant variable ret in cleancache_get_page().

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>