fs: add a vfs_fchmod helper

[ Upstream commit 9e96c8c0e94eea2f69a9705f5d0f51928ea26c17 ]

Add a helper for struct file based chmode operations.  To be used by
the initramfs code soon.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Stable-dep-of: 4624b346cf67 ("init: open /initrd.image with O_LARGEFILE")
Signed-off-by: Sasha Levin <sashal@kernel.org>