btrfs: handle ACLs on idmapped mounts

Make the ACL code idmapped mount aware. The POSIX default and POSIX
access ACLs are the only ACLs other than some specific xattrs that take
DAC permissions into account. On an idmapped mount they need to be
translated according to the mount's userns. The main change is done to
__btrfs_set_acl() which is responsible for translating POSIX ACLs to
their final on-disk representation.

The btrfs_init_acl() helper does not need to take the idmapped mount
into account since it is called in the context of file creation
operations (mknod, create, mkdir, symlink, tmpfile) and is used for
btrfs_init_inode_security() to copy POSIX default and POSIX access
permissions from the parent directory. These ACLs need to be inherited
unmodified from the parent directory. This is identical to what we do
for ext4 and xfs.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>