acl: make posix_acl_clone() available to overlayfs

The ovl_get_acl() function needs to alter the POSIX ACLs retrieved from the
lower filesystem. Instead of hand-rolling a overlayfs specific
posix_acl_clone() variant allow export it. It's not special and it's not deeply
internal anyway.

Link: https://lore.kernel.org/r/20220708090134.385160-3-brauner@kernel.org
Cc: Seth Forshee <sforshee@digitalocean.com>
Cc: Amir Goldstein <amir73il@gmail.com>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Aleksa Sarai <cyphar@cyphar.com>
Cc: Miklos Szeredi <mszeredi@redhat.com>
Cc: linux-unionfs@vger.kernel.org
Cc: linux-fsdevel@vger.kernel.org
Reviewed-by: Seth Forshee <sforshee@digitalocean.com>
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>

fs/posix_acl.c

@@ -199,7 +199,7 @@ EXPORT_SYMBOL(posix_acl_alloc);
 /*
  * Clone an ACL.
  */
-static struct posix_acl *
+struct posix_acl *
 posix_acl_clone(const struct posix_acl *acl, gfp_t flags)
 {
 	struct posix_acl *clone = NULL;
@@ -213,6 +213,7 @@ posix_acl_clone(const struct posix_acl *acl, gfp_t flags)
 	}
 	return clone;
 }
+EXPORT_SYMBOL_GPL(posix_acl_clone);
 
 /*
  * Check if an acl is valid. Returns 0 if it is, or -E... otherwise.

include/linux/posix_acl.h

@@ -73,6 +73,7 @@ extern int set_posix_acl(struct user_namespace *, struct inode *, int,
 			 struct posix_acl *);
 
 struct posix_acl *get_cached_acl_rcu(struct inode *inode, int type);
+struct posix_acl *posix_acl_clone(const struct posix_acl *acl, gfp_t flags);
 
 #ifdef CONFIG_FS_POSIX_ACL
 int posix_acl_chmod(struct user_namespace *, struct inode *, umode_t);