fs: move code out of buffer.c

Move invalidate_bdev, block_sync_page into fs/block_dev.c.  Export
kill_bdev as well, so brd doesn't have to open code it.  Reduce
buffer_head.h requirement accordingly.

Removed a rather large comment from invalidate_bdev, as it looked a bit
obsolete to bother moving.  The small comment replacing it says enough.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
index 43db715..7867b7c 100644
--- a/drivers/tty/sysrq.c
+++ b/drivers/tty/sysrq.c
@@ -32,7 +32,6 @@
 #include <linux/module.h>
 #include <linux/suspend.h>
 #include <linux/writeback.h>
-#include <linux/buffer_head.h>		/* for fsync_bdev() */
 #include <linux/swap.h>
 #include <linux/spinlock.h>
 #include <linux/vt_kern.h>
@@ -41,6 +40,7 @@
 #include <linux/oom.h>
 #include <linux/slab.h>
 #include <linux/input.h>
+#include <linux/uaccess.h>
 
 #include <asm/ptrace.h>
 #include <asm/irq_regs.h>