CIFS: SMBD: work around gcc -Wmaybe-uninitialized warning

GCC versions from 4.9 to 6.3 produce a false-positive warning when
dealing with a conditional spin_lock_irqsave():

fs/cifs/smbdirect.c: In function 'smbd_recv_buf':
include/linux/spinlock.h:260:3: warning: 'flags' may be used uninitialized in this function [-Wmaybe-uninitialized]

This function calls some sleeping interfaces, so it is clear that it
does not get called with interrupts disabled and there is no need
to save the irq state before taking the spinlock. This lets us
remove the variable, which makes the function slightly more efficient
and avoids the warning.

A further cleanup could do the same change for other functions in this
file, but I did not want to take this too far for now.

Fixes: ac69f66e54ca ("CIFS: SMBD: Implement function to receive data via RDMA receive")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Steve French <smfrench@gmail.com>