ocfs2: Check search result in ocfs2_xattr_block_get()

ocfs2_xattr_block_get() calls ocfs2_xattr_search() to find an external
xattr, but doesn't check the search result that is passed back via struct
ocfs2_xattr_search. Add a check for search result, and pass back -ENODATA if
the xattr search failed. This avoids a later NULL pointer error.

Signed-off-by: Tiger Yang <tiger.yang@oracle.com>
Signed-off-by: Mark Fasheh <mfasheh@suse.com>