afs: Fix incorrect error handling in afs_xattr_get_acl()

Fix incorrect error handling in afs_xattr_get_acl() where there appears to
be a redundant assignment before return, but in fact the return should be a
goto to the error handling at the end of the function.

Fixes: 260f082b ("afs: Get an AFS3 ACL as an xattr")
Addresses-Coverity: ("Unused Value")
Reported-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Joe Perches <joe@perches.com>