libfs: Add simple_offset_rename() API

libfs: Add simple_offset_rename() API

[ Upstream commit 5a1a25be995e1014abd01600479915683e356f5c ]

I'm about to fix a tmpfs rename bug that requires the use of
internal simple_offset helpers that are not available in mm/shmem.c

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Link: https://lore.kernel.org/r/20240415152057.4605-3-cel@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>
Stable-dep-of: ad191eb6d694 ("shmem: Fix shmem_rename2()")
Signed-off-by: Sasha Levin <sashal@kernel.org>