This was spotted by automated code analysis. In case reading
an ACL xattr failed (only likely to happen if there is an I/O
error for example, and even then only with unstuffed xattrs,
so pretty difficult to trigger) a small amount of memory could
potentially be leaked.

This patch adds a kfree to the error path, and also removes a
test which is no longer required (gfs2_ea_get_copy always
returns either a negative error, or a length)
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>