[REGRESSION] fuse: execve() fails with ETXTBSY due to async fuse_flush

[Jürg Billeter]

Since v6.3-rc1 commit 5a8bee63b1 ("fuse: in fuse_flush only wait if
someone wants the return code") `fput()` is called asynchronously if a
file is closed as part of a process exiting, i.e., if there was no
explicit `close()` before exit.

If the file was open for writing, also `put_write_access()` is called
asynchronously as part of the async `fput()`.

If that newly written file is an executable, attempting to `execve()`
the new file can fail with `ETXTBSY` if it's called after the writer
process exited but before the async `fput()` has run.

I've confirmed that this issue is absent in v6.2 and reverting
5a8bee63b1 on top of v6.4.10 fixes the regression.

#regzbot introduced: 5a8bee63b1

Jürg