syscall: match seek argument size to signature

Caught by go vet:
asm_linux_arm.s:110: [arm] seek: wrong argument size 32; expected $...-28

Change-Id: I62ec5327a25bff9ef501c42cc1e28ea7ec78510f
Reviewed-on: https://go-review.googlesource.com/2810Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/syscall/asm_linux_arm.s

@@ -107,7 +107,7 @@ ok2:
 // taking the address of the return value newoffset.
 // Underlying system call is
 //	llseek(int fd, int offhi, int offlo, int64 *result, int whence)
-TEXT ·seek(SB),NOSPLIT,$0-32
+TEXT ·seek(SB),NOSPLIT,$0-28
 	BL	runtime·entersyscall(SB)
 	MOVW	$SYS__LLSEEK, R7	// syscall entry
 	MOVW	4(SP), R0	// fd