There are a number of places where generic_file_write could be used if
it didn't take i_sem by itself or where it doesn't need it:

(1)	XFS needs to do some preparation in it's write file operation
	(most notably taking it's own per-inode locks), after taking
	i_sem, but before calling the guts of generic_file_write.
(2)	blockdevices are happy with parralel get_block() operations,
	and there is nothing i_sem could protect.

This patch introduces a generic_file_write_nolock and makes the good old
generic_file_write a wrapper around.  The block device code is switched
over to it.