NFS: clean up a needless assignment in nfs_file_write()

Commit 064109db ("NFS: remove redundant code in nfs_file_write()")
identifies that filemap_fdatawait_range() will always return 0 and removes
a dead error-handling case in nfs_file_write(). With this change however,
assigning the return of filemap_fdatawait_range() to the result variable is
a dead store.

Remove this needless assignment.

No functional change. No change in object code.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>