CIFS: Reopen file before get SMB2 MTU credits for async IO

Currently we get MTU credits before we check an open file if
it needs to be reopened. Reopening the file in such conditions
leads to a possibility of being stuck waiting indefinitely
for credits in the transport layer. Fix this by reopening the
file first if needed and then getting MTU credits for async IO.

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