vfs_shadow_copy2: Preserve all open flags assuming ROFS

vfs_shadow_copy2: Preserve all open flags assuming ROFS

Instead of replacing open flags with just O_RDONLY, filter out all those
flags unrelated to a Read Only File System

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14573

Signed-off-by: Anoop C S <anoopcs@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>
Autobuild-User(master): Ralph Böhme <slow@samba.org>
Autobuild-Date(master): Thu Nov 12 17:23:19 UTC 2020 on sn-devel-184

(cherry picked from commit e9e06a11daf036abf7a7022ebc8eaefde178aa52)

Autobuild-User(v4-12-test): Karolin Seeger <kseeger@samba.org>
Autobuild-Date(v4-12-test): Thu Nov 19 14:09:43 UTC 2020 on sn-devel-184