ocfs2: strict bound check before memcmp in ocfs2_xattr_find_entry()

In the Linux kernel, the following vulnerability has been resolved: ocfs2: strict bound check before memcmp in ocfs2_xattr_find_entry() xattr in ocfs2 maybe 'non-indexed', which saved with additional space requested. It's better to check if the memory is out of bound before memcmp, although this possibility mainly comes from crafted poisonous images.