tty: fix BKL related leak and crash

Enabling the BKL to be lockdep tracked uncovered the following
upstream kernel bug in the tty code, which caused a BKL
reference leak:

  ================================================
  [ BUG: lock held when returning to user space! ]
  ------------------------------------------------
  dmesg/3121 is leaving the kernel with locks still held!
  1 lock held by dmesg/3121:
   #0:  (kernel_mutex){--..}, at: [<c02f34d9>] opost+0x24/0x194

this might explain some of the atomicity warnings and crashes
that -tip tree testing has been experiencing since the BKL
was converted back to a spinlock.
Signed-off-by: NIngo Molnar <mingo@elte.hu>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

drivers/char/n_tty.c

@@ -282,16 +282,20 @@ static int opost(unsigned char c, struct tty_struct *tty)
 			if (O_ONLRET(tty))
 				tty->column = 0;
 			if (O_ONLCR(tty)) {
-				if (space < 2)
+				if (space < 2) {
+					unlock_kernel();
 					return -1;
+				}
 				tty_put_char(tty, '\r');
 				tty->column = 0;
 			}
 			tty->canon_column = tty->column;
 			break;
 		case '\r':
-			if (O_ONOCR(tty) && tty->column == 0)
+			if (O_ONOCR(tty) && tty->column == 0) {
+				unlock_kernel();
 				return 0;
+			}
 			if (O_OCRNL(tty)) {
 				c = '\n';
 				if (O_ONLRET(tty))
@@ -303,10 +307,13 @@ static int opost(unsigned char c, struct tty_struct *tty)
 		case '\t':
 			spaces = 8 - (tty->column & 7);
 			if (O_TABDLY(tty) == XTABS) {
-				if (space < spaces)
+				if (space < spaces) {
+					unlock_kernel();
 					return -1;
+				}
 				tty->column += spaces;
 				tty->ops->write(tty, "        ", spaces);
+				unlock_kernel();
 				return 0;
 			}
 			tty->column += spaces;