iomap: remove pos+len BUG_ON() to after folio lookup

iomap: remove pos+len BUG_ON() to after folio lookup

The bug checks at the top of iomap_write_begin() assume the pos/len
reflect exactly the next range to process. This may no longer be the
case once the get folio path is able to process a folio batch from
the filesystem. On top of that, len is already trimmed to within the
iomap/srcmap by iomap_length(), so these checks aren't terribly
useful. Remove the unnecessary BUG_ON() checks.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>