staging: erofs: drop the extern prefix for function definitions

Fix all `CHECK: extern prototypes should be avoided in .h files'
reported by checkpatch.pl.

Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>