Making linkat() able to overwrite the target

[David Howells]

With my rewrite of fscache and cachefiles:

	https://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git/log/?h=fscache-iter

when a file gets invalidated by the server - and, under some circumstances,
modified locally - I have the cache create a temporary file with vfs_tmpfile()
that I'd like to just link into place over the old one - but I can't because
vfs_link() doesn't allow you to do that.  Instead I have to either unlink the
old one and then link the new one in or create it elsewhere and rename across.

Would it be possible to make linkat() take a flag, say AT_LINK_REPLACE, that
causes the target to be replaced and not give EEXIST?  Or make it so that
rename() can take a tmpfile as the source and replace the target with that.  I
presume that, either way, this would require journal changes on ext4, xfs and
btrfs.

Thanks,
David