staging: erofs: kill CONFIG_EROFS_FS_USE_VM_MAP_RAM

Turn into a module parameter ("use_vmap") as it
can be set at runtime.

Suggested-by: David Sterba <dsterba@suse.cz>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Link: https://lore.kernel.org/r/20190731155752.210602-15-gaoxiang25@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>