KVM: guest_memfd: pass error up from filemap_grab_folio

Some SNP ioctls will require the page not to be in the pagecache, and as such they
will want to return EEXIST to userspace.  Start by passing the error up from
filemap_grab_folio.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

virt/kvm/guest_memfd.c

@@ -19,8 +19,8 @@ static struct folio *kvm_gmem_get_folio(struct inode *inode, pgoff_t index)
 
 	/* TODO: Support huge pages. */
 	folio = filemap_grab_folio(inode->i_mapping, index);
-	if (IS_ERR_OR_NULL(folio))
-		return NULL;
+	if (IS_ERR(folio))
+		return folio;
 
 	/*
 	 * Use the up-to-date flag to track whether or not the memory has been
@@ -146,8 +146,8 @@ static long kvm_gmem_allocate(struct inode *inode, loff_t offset, loff_t len)
 		}
 
 		folio = kvm_gmem_get_folio(inode, index);
-		if (!folio) {
-			r = -ENOMEM;
+		if (IS_ERR(folio)) {
+			r = PTR_ERR(folio);
 			break;
 		}
 
@@ -505,8 +505,8 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
 	}
 
 	folio = kvm_gmem_get_folio(file_inode(file), index);
-	if (!folio) {
-		r = -ENOMEM;
+	if (IS_ERR(folio)) {
+		r = PTR_ERR(folio);
 		goto out_fput;
 	}