iomap: Add another assertion to inline data handling

Check that the file tail does not cross a page boundary.  Requested by
Andreas.

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