FS: cifs, remove unneeded NULL tests

Stanse found that pSMBFile in cifs_ioctl and file->f_path.dentry in
cifs_user_write are dereferenced prior their test to NULL.

The alternative is not to dereference them before the tests. The patch is
to point out the problem, you have to decide.

While at it we cache the inode in cifs_user_write to a local variable
and use all over the function.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Steve French <sfrench@samba.org>
Cc: linux-cifs@vger.kernel.org
Cc: Jeff Layton <jlayton@redhat.com>
Cc: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Steve French <sfrench@us.ibm.com>