NFS: Fix O_DIRECT compile warnings

Fix the following compile warnings:
fs/nfs/direct.c: In function 'nfs_direct_read_schedule_segment':
fs/nfs/direct.c:325:11: warning: comparison of distinct pointer types
lacks a cast [enabled by default]
fs/nfs/direct.c:325:11: warning: comparison of distinct pointer types
lacks a cast [enabled by default]
fs/nfs/direct.c:325:11: warning: comparison of distinct pointer types
lacks a cast [enabled by default]
fs/nfs/direct.c:352:27: warning: comparison of distinct pointer types
lacks a cast [enabled by default]
fs/nfs/direct.c: In function 'nfs_direct_write_schedule_segment':
fs/nfs/direct.c:622:11: warning: comparison of distinct pointer types
lacks a cast [enabled by default]
fs/nfs/direct.c:622:11: warning: comparison of distinct pointer types
lacks a cast [enabled by default]
fs/nfs/direct.c:622:11: warning: comparison of distinct pointer types
lacks a cast [enabled by default]
fs/nfs/direct.c:650:27: warning: comparison of distinct pointer types
lacks a cast [enabled by default]

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Cc: Fred Isaman <iisaman@netapp.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>