Introduce new function for generic inode syncing (vfs_fsync_range) and use
it from fsync() path. Introduce also new helper for syncing after a sync
write (generic_write_sync) using the generic function.

Use these new helpers for syncing from generic VFS functions. This makes
O_SYNC writes to block devices acquire i_mutex for syncing. If we really
care about this, we can make block_fsync() drop the i_mutex and reacquire
it before it returns.

CC: Evgeniy Polyakov <zbr@ioremap.net>
CC: ocfs2-devel@oss.oracle.com
CC: Joel Becker <joel.becker@oracle.com>
CC: Felix Blyakher <felixb@sgi.com>
CC: xfs@oss.sgi.com
CC: Anton Altaparmakov <aia21@cantab.net>
CC: linux-ntfs-dev@lists.sourceforge.net
CC: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
CC: linux-ext4@vger.kernel.org
CC: tytso@mit.edu
Acked-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jan Kara <jack@suse.cz>