ext4: iomap that extends beyond EOF should be marked dirty

This patch addresses what Dave Chinner had discovered and fixed within
commit: 7684e2c4. This changes does not have any user visible
impact for ext4 as none of the current users of ext4_iomap_begin()
that extend files depend on IOMAP_F_DIRTY.

When doing a direct IO that spans the current EOF, and there are
written blocks beyond EOF that extend beyond the current write, the
only metadata update that needs to be done is a file size extension.

However, we don't mark such iomaps as IOMAP_F_DIRTY to indicate that
there is IO completion metadata updates required, and hence we may
fail to correctly sync file size extensions made in IO completion when
O_DSYNC writes are being used and the hardware supports FUA.

Hence when setting IOMAP_F_DIRTY, we need to also take into account
whether the iomap spans the current EOF. If it does, then we need to
mark it dirty so that IO completion will call generic_write_sync() to
flush the inode size update to stable storage correctly.

Signed-off-by: Matthew Bobrowski <mbobrowski@mbobrowski.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Ritesh Harjani <riteshh@linux.ibm.com>
Link: https://lore.kernel.org/r/8b43ee9ee94bee5328da56ba0909b7d2229ef150.1572949325.git.mbobrowski@mbobrowski.org


Signed-off-by: Theodore Ts'o <tytso@mit.edu>