riscv: fix locking violation in page fault handler

When a user mode process accesses an address in the vmalloc area
do_page_fault tries to unlock the mmap semaphore when it isn't locked.

Signed-off-by: Andreas Schwab <schwab@suse.de>
[Palmer: Duplicated code instead of a goto]
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>

arch/riscv/mm/fault.c

@@ -229,8 +229,9 @@ vmalloc_fault:
 		pte_t *pte_k;
 		int index;
 
+		/* User mode accesses just cause a SIGSEGV */
 		if (user_mode(regs))
-			goto bad_area;
+			return do_trap(regs, SIGSEGV, code, addr, tsk);
 
 		/*
 		 * Synchronize this task's top level page-table