[CIFS] Allow local filesize for file that is open for write to be updated

[CIFS] Allow local filesize for file that is open for write to be updated
from server when mount forcedirectio.

Allowing update of file size with non forcedirectio mounts should be
allowed in the fiture but requires carefully writing out the
last page in the local file if it is a partial page in order to
avoid corruption and careful serialization

Thanks to Maximiliano Curia who suggested similar changes and provided
a testcase.

Signed-off-by: Steve French <sfrench@us.ibm.com>