GFS2: rgrp free blocks used incorrectly

GFS2: rgrp free blocks used incorrectly

Before this patch, several functions in rgrp.c checked the value of
rgd->rd_free_clone. That does not take into account blocks that were
reserved by a multi-block reservation. This causes a problem when
space gets tight in the file system. For example, when function
gfs2_inplace_reserve checks to see if a rgrp has enough blocks to
satisfy the request, it can accept a rgrp that it should reject
because, although there are enough blocks to satisfy the request
_now_, those blocks may be reserved for another running process.

A second problem with this occurs when we've reserved the remaining
blocks in an rgrp: function rg_mblk_search() can reject an rgrp
improperly because it calculates:

   u32 free_blocks = rgd->rd_free_clone - rgd->rd_reserved;

But rd_reserved includes blocks that the current process just
reserved in its own call to inplace_reserve. For example, it can
reserve the last 128 blocks of an rgrp, then reject that same rgrp
because the above calculates out to free_blocks = 0;

Consequences include, but are not limited to, (1) leaving holes,
and thus increasing file system fragmentation, and (2) reporting
file system is full long before it actually is.

This patch introduces a new function, rgd_free, which returns the
number of clone-free blocks (blocks that are truly free as opposed
to blocks that are still being used because an unlinked file is
still open) minus the number of blocks reserved by processes, but
not counting the blocks we ourselves reserved (because obviously
we need to allocate them).

Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>