NFSv4: Check for buffer length in __nfs4_get_acl_uncached

NFSv4: Check for buffer length in __nfs4_get_acl_uncached

commit 7d3e91a89b7adbc2831334def9e494dd9892f9af upstream.

Commit 1f1ea6c "NFSv4: Fix buffer overflow checking in
__nfs4_get_acl_uncached" accidently dropped the checking for too small
result buffer length.

If someone uses getxattr on "system.nfs4_acl" on an NFSv4 mount
supporting ACLs, the ACL has not been cached and the buffer suplied is
too short, we still copy the complete ACL, resulting in kernel and user
space memory corruption.

Signed-off-by: Sven Wegener <sven.wegener@stealer.net>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>