CIFS: Adjust MTU credits before reopening a file

Currently we adjust MTU credits before sending an IO request
and after reopening a file. This approach doesn't allow the
reopen routine to use existing credits that are not needed
for IO. Reorder credit adjustment and reopening a file to
use credits available to the client more efficiently. Also
unwrap complex if statement into few pieces to improve
readability.

Signed-off-by: Pavel Shilovsky <pshilov@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>