cifs: dereferencing first then checking

This patch is based on Dan's original patch. His original description is
below:

Smatch complained about a couple checking for NULL after dereferencing
bugs.  I'm not super familiar with the code so I did the conservative
thing and move the dereferences after the checks.

The dereferences in cifs_lock() and cifs_fsync() were added in
ba00ba64 "cifs: make various routines use the cifsFileInfo->tcon
pointer".  The dereference in find_writable_file() was added in
6508d904 "cifs: have find_readable/writable_file filter by fsuid".
The comments there say it's possible to trigger the NULL dereference
under stress.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>