fs: Fix page_mkwrite off-by-one errors

The check in block_page_mkwrite that is meant to determine whether an
offset is within the inode size is off by one.  This bug has been copied
into iomap_page_mkwrite and several filesystems (ubifs, ext4, f2fs,
ceph).

Fix that by introducing a new page_mkwrite_check_truncate helper that
checks for truncate and computes the bytes in the page up to EOF.  Use
the helper in iomap.

NOTE from Darrick: The original patch fixed a number of filesystems, but
then there were merge conflicts with the f2fs for-next tree; a
subsequent re-submission of the patch had different btrfs changes with
no explanation; and Christoph complained that each per-fs fix should be
a separate patch.  In my view that's too much risk to take on, so I
decided to drop all the hunks except for iomap, since I've actually QA'd
XFS.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
[darrick: drop everything but the iomap parts]
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>