nfs: Fix unused variable warning from file.c

Fix the following unused variable warning.

fs/nfs/file.c: In function ‘nfs_file_release’:
fs/nfs/file.c:140:17: warning: unused variable ‘dentry’
fs/nfs/file.c: In function ‘nfs_file_read’:
fs/nfs/file.c:237:9: warning: unused variable ‘count’

Signed-off-by: Rakib Mullick <rakib.mullick@gmail.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>