cmfs: several bug fixes in cmfs mount code

this patch fixes several minor bugs in cmfs mount code,
- in cmfs_get_sector(), add unlock_buffer() before calling ll_rw_block().
- add cmfs_recovery_init(), which intializes several veraiables.
- in cmfs_initialize_super(), seet sb->s_fs_info by csb before calling CMFS_HAS_COMPAT_FEATURE(), because sb->s_fs_info is referenced in this macro.