AFS: write support fixes

AFS write support fixes:

 (1) Support large files using the 64-bit file access operations if available
     on the server.

 (2) Use kmap_atomic() rather than kmap() in afs_prepare_page().

 (3) Don't do stuff in afs_writepage() that's done by the caller.

[akpm@linux-foundation.org: fix right shift count >= width of type]
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>