x86/sgx: Handle VA page allocation failure for EAUG on PF.

VM_FAULT_NOPAGE is expected behaviour for -EBUSY failure path, when
augmenting a page, as this means that the reclaimer thread has been
triggered, and the intention is just to round-trip in ring-3, and
retry with a new page fault.

Fixes: 5a90d2c3f5ef ("x86/sgx: Support adding of pages to an initialized enclave")
Signed-off-by: Haitao Huang <[e-mail]>
Signed-off-by: Jarkko Sakkinen <[e-mail]>
Signed-off-by: Dave Hansen <[e-mail]>
Reviewed-by: Reinette Chatre <[e-mail]>
Tested-by: Vijay Dhanraj <[e-mail]>
Cc: [e-mail]
Link: https://lkml.kernel.org/r/[e-mail]

arch/x86/kernel/cpu/sgx/encl.c

···
 	}
 
 	va_page = sgx_encl_grow(encl, false);
-	if (IS_ERR(va_page))
+	if (IS_ERR(va_page)) {
+		if (PTR_ERR(va_page) == -EBUSY)
+			vmret = VM_FAULT_NOPAGE;
 		goto err_out_epc;
+	}
 
 	if (va_page)
 		list_add(&va_page->list, &encl->va_pages);