ocfs2: Check errors from ocfs2_xattr_update_xattr_search()

The ocfs2_xattr_update_xattr_search() function can return an error when
trying to read blocks off of disk.  The caller needs to check this error
before using those (possibly invalid) blocks.

Signed-off-by: Joel Becker <joel.becker@oracle.com>
Signed-off-by: Mark Fasheh <mfasheh@suse.com>