gfs2: Fix gfs2_release for non-writers regression

When a file is opened for writing, the vfs code (do_dentry_open)
calls get_write_access for the inode, thus incrementing the inode's write
count. That writer normally then creates a multi-block reservation for
the inode (i_res) that can be re-used by other writers, which speeds up
writes for applications that stupidly loop on open/write/close.
When the writes are all done, the multi-block reservation should be
deleted when the file is closed by the last "writer."

Commit 0ec9b9ea broke that concept when it moved the call to
gfs2_rs_delete before the check for FMODE_WRITE.  Non-writers have no
business removing the multi-block reservations of writers. In fact, if
someone opens and closes the file for RO while a writer has a
multi-block reservation, the RO closer will delete the reservation
midway through the write, and this results in:

kernel BUG at fs/gfs2/rgrp.c:677! (or thereabouts) which is:
BUG_ON(rs->rs_requested); from function gfs2_rs_deltree.

This patch moves the check back inside the check for FMODE_WRITE.

Fixes: 0ec9b9ea

 ("gfs2: Check for active reservation in gfs2_release")
Cc: stable@vger.kernel.org # v5.12+
Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>