syscall: remove calls to Entersyscall & Exitsyscall in Getdents

    
    The syscall for Getdents in syscall/libcall_linux.go
    called Entersyscall and Exitsyscall, causing the
    runtime_sched counts for goroutines to be incorrect.
    Inconsistent counts caused checkdead in runtime/proc.c
    to panic.
    
    Fixes golang/go#11406
    
    Reviewed-on: https://go-review.googlesource.com/11761

From-SVN: r225745