ext4: add missing error check in __ext4_new_inode()

ext4: add missing error check in __ext4_new_inode()

commit 996fc4477a0ea28226b30d175f053fb6f9a4fa36 upstream.

It's possible for ext4_get_acl() to return an ERR_PTR.  So we need to
add a check for this case in __ext4_new_inode().  Otherwise on an
error we can end up oops the kernel.

This was getting triggered by xfstests generic/388, which is a test
which exercises the shutdown code path.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>