ext4: avoid unneeded lookup when xattr name is invalid

In ext4_xattr_set_handle() we have checked the xattr name's length. So
we should also check it in ext4_xattr_get() to avoid unneeded lookup
caused by invalid name.

Signed-off-by: Zhang Zhen <zhenzhang.zhang@huawei.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>