do_cifs_create(): don't set ->i_mode of something we had not created

If the file had existed before we'd called ->atomic_open() (without
O_EXCL, that is), we have no more business setting ->i_mode than
we would setting ->i_uid or ->i_gid.  We also have no business
doing either if another client has managed to get unlink+mkdir
between ->open() and cifs_inode_get_info().

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>