Btrfs: igrab inode in writepage

commit be7bd730841e69fe8f70120098596f648cd1f3ff upstream.

We hit this panic on a few of our boxes this week where we have an
ordered_extent with an NULL inode.  We do an igrab() of the inode in writepages,
but weren't doing it in writepage which can be called directly from the VM on
dirty pages.  If the inode has been unlinked then we could have I_FREEING set
which means igrab() would return NULL and we get this panic.  Fix this by trying
to igrab in btrfs_writepage, and if it returns NULL then just redirty the page
and return AOP_WRITEPAGE_ACTIVATE; so the VM knows it wasn't successful.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fb.com>
Reviewed-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>