libafs: Remove afs_write duplication

libafs: Remove afs_write duplication

The afs_write() code for memory and disk cache suffered from exactly
the same duplication problems as the afs_read() code.

Apply a similar fix - unify afs_UFSWrite and afs_MemWrite into a single
afs_write function, place the UFS specific code into afs_UFSWriteUIO,
and make use of the existing afs_MemWriteUIO for the memcache case.

Change-Id: I074e1f56597e5cf04d13a45bcda5ad5fedb6377f
Reviewed-on: http://gerrit.openafs.org/4465
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>