GFS2: Fix very unlikley memory leak in ACL xattr code

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>