eCryptfs: convert mmap functions to use persistent file

Convert readpage, prepare_write, and commit_write to use read_write.c
routines.  Remove sync_page; I cannot think of a good reason for implementing
that in eCryptfs.

Signed-off-by: Michael Halcrow <mhalcrow@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>