iomap: Fix some typos and bad grammar

Fix some typos and bad grammar in buffered-io.c to make the comments
easier to read.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>