CVE-2026-53244: VFS: fix possible failure to unlock in nfsd4_create_file()

In the Linux kernel, the following vulnerability has been resolved:

VFS: fix possible failure to unlock in nfsd4createfile()

atomiccreate() in fs/namei.c drops the reference to the dentry when it returns an error. This behaviour was imported into dentrycreate() so that it will drop the reference if an error is returned from atomiccreate(), though not if vfscreate() returns an error (in the case where ->atomiccreate is not supported).

The caller - nfsd4createfile() - is made aware of this by checking path->dentry, which will either be a counted reference to a dentry, or an error pointer.

However the change to use startcreating()/endcreating() (which landed shortly before the dentrycreate() change landed, though was likely developed around the same time) means that nfsd4createfile() needs a valid dentry so that it can unlock the parent.

The net result is that if NFSD exports a filesystem which uses ->atomiccreate, and if a call to ->atomiccreate returns an error, then nfsd4createfile() will pass an error pointer to endcreating() and the parent will not be unlocked.

Fix this by changing dentrycreate() to make sure path->dentry is always a valid dentry, never an error-pointer. The actual error is already returned a different way.

Note that if ->atomiccreate() returns a different dentry (which may not be possible in practice) we are guaranteed (because it is only ever provided by dspliacealias()) that it will have the same dparent and so it will have the same effect when passed to endcreating().