fixing redundant network opens on Linux file creation

[Bryan Henderson]

>There is a lookup intent patch from lustre group. It can be found
>somewhere in the archives. Pushing that (or something along that lines)
>to mainline and using that would be IMHO most beneficial

Better still would be to add a "create-and-open" VFS call and have namei
use it.  This solves a number of problems, including the fact that it is
impossible to correctly implement an exclusive create and open with a
shared filesystem (because between when Linux confirms that the file
doesn't exist and when Linux does the VFS create, another system may have
created the file).

"Intent," as it's generally understood, is not a promise of future activity
-- it's either a hint to improve efficiency or it's a promise to restrict
future activity, but it should be possible simply to bail out before
exercising that intent.  E.g. you can't open a file at the same time as it
is looked up just because the looker upper says he intends to open it
later.