nfsd: in nfsd4_create_file() let VFS report if file was created.

nfsd4_create_file() currently assumes that if a lookup failed but then a
create succeeds, then the "create" operation actually created the file.
With atomic_open this may not be the case - some other actor might have
created the file between the lookup and the create.

So we move the call to nfsd4_vfs_create() earlier and set ->op_created
based on the FMODE_CREATED flag that it set.  Then use "!  ->op_created"
to trigger nfserr_exist handling.

The switch statement is split up into two if() statements.
First we check for the possibility of a successful exclusive
create and set ->op_create to true if appropriate.
Then we check for NFS4_CREATE_UNCHECKED to decide if a
pre-existing file means an error or success.

This allows us to combine the two fh_compose() calls to one place.

A subtle difference here is that we now must only pass O_EXCL to
dentry_create() for NFS4_CREATE_GUARDED.  For the EXCLUSIVE create modes
we want a successful open even if the file already exists.  We then
check the verifier after the open succeeded to see if it was exclusive.

The above requires changing dentry_create() to reliably set
FMODE_CREATED when the file was actually created.  Previously it only
sets this flag when atomic_open is used.

Signed-off-by: NeilBrown <neil@brown.name>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20260717093001.1972119-9-neilb@ownmail.net
Signed-off-by: Chuck Lever <cel@kernel.org>

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