libfs: Define a minimum directory offset

libfs: Define a minimum directory offset

[ Upstream commit 7beea725a8ca412c6190090ce7c3a13b169592a1 ]

This value is used in several places, so make it a symbolic
constant.

Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Link: https://lore.kernel.org/r/170820142741.6328.12428356024575347885.stgit@91.116.238.104.host.secureserver.net
Signed-off-by: Christian Brauner <brauner@kernel.org>
Stable-dep-of: 23cdd0eed3f1 ("libfs: Fix simple_offset_rename_exchange()")
Signed-off-by: Sasha Levin <sashal@kernel.org>