ext4: remove unneeded variable in ext4_xattr_check_block()

We could return directly from ext4_xattr_check_block(). Thus, we
shouldn't need to define a 'error' variable.

Signed-off-by: Zheng Liu <wenqing.lz@taobao.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>