EX-8993 ofd: do not write 'hole' pages on compression

EX-8993 ofd: do not write 'hole' pages on compression

When doing unaligned read-modify-write to a compressed file,
we must round the IO lnb used for write in order to read up
the compressed data for modification.

In some cases, this creates a situation where there are
pages in the write lnb which have no data in them.  It is
important not to write out these pages, because if we do,
this wastes space and can cause incorrect file size.

In most cases, the file size is covered by the client
sending the file size, but if the client does not compress
a particular write, it does not send the size and the server
does not use it.  We could resolve this by having the client
always send size info and have the server always use it, but
it's better to make server writes 'hole' aware, since this
improves space usage.  (And this will be required for the
server to do recompression on read-modify-write, otherwise
no space is gained.)

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I66169e205fe4691ed03b2c9b3005ffc4ecd3213d
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/53595
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Artem Blagodarenko <ablagodarenko@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>