cifsd: fix invalid memory access in smb2_write()

Add missing fp initialzation to prevent invalid memory access in
smb2_write().

Reported-by: Coverity Scan <scan-admin@coverity.com>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Steve French <stfrench@microsoft.com>