afs: Fix directory entry size calculation

The number of dirent records used by an AFS directory entry should be
calculated using the assumption that there is a 16-byte name field in the
first block, rather than a 20-byte name field (which is actually the case).
This miscalculation is historic and effectively standard, so we have to use
it.

The calculation we need to use is:

	1 + (((strlen(name) + 1) + 15) >> 5)

where we are adding one to the strlen() result to account for the NUL
termination.

Fix this by the following means:

 (1) Create an inline function to do the calculation for a given name
     length.

 (2) Use the function to calculate the number of records used for a dirent
     in afs_dir_iterate_block().

     Use this to move the over-end check out of the loop since it only
     needs to be done once.

     Further use this to only go through the loop for the 2nd+ records
     composing an entry.  The only test there now is for if the record is
     allocated - and we already checked the first block at the top of the
     outer loop.

 (3) Add a max name length check in afs_dir_iterate_block().

 (4) Make afs_edit_dir_add() and afs_edit_dir_remove() use the function
     from (1) to calculate the number of blocks rather than doing it
     incorrectly themselves.

Fixes: 63a4681f ("afs: Locally edit directory data for mkdir/create/unlink/...")
Fixes: ^1da177e4 ("Linux-2.6.12-rc2")
Signed-off-by: David Howells <dhowells@redhat.com>
Tested-by: Marc Dionne <marc.dionne@auristor.com>