on non-posix shares, clear write bits in mode when ATTR_READONLY is set

When mounting a share with posix extensions disabled,
cifs_get_inode_info turns off all the write bits in the mode for regular
files if ATTR_READONLY is set. Directories and other inode types,
however, can also have ATTR_READONLY set, but the mode gives no
indication of this.

This patch makes this apply to other inode types besides regular files.
It also cleans up how modes are set in cifs_get_inode_info for both the
"normal" and "dynperm" cases.

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