iomap: Allow iomap_write_begin() to be called with the full length

In the future, we want write_begin to know the entire length of the
write so that it can choose to allocate large folios.  Pass the full
length in from __iomap_zero_iter() and limit it where necessary.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>