afs: Fix setting of i_blocks

The setting of i_blocks, which is calculated from i_size, has got
accidentally misordered relative to the setting of i_size when initially
setting up an inode.  Further, i_blocks isn't updated by afs_apply_status()
when the size is updated.

To fix this, break the i_size/i_blocks setting out into a helper function
and call it from both places.

Fixes: a58823ac ("afs: Fix application of status and callback to be under same lock")
Signed-off-by: David Howells <dhowells@redhat.com>