gfs2: gl_object races fix

gfs2: gl_object races fix

Function glock_clear_object() checks if the specified glock is still
pointing at the right object and clears the gl_object pointer.  To
handle the case of incompletely constructed inodes, glock_clear_object()
also allows gl_object to be NULL.

However, in the teardown case, when iget_failed() is called and the
inode is removed from the inode hash, by the time we get to the
glock_clear_object() calls in gfs2_put_super() and its helpers, we don't
have exclusion against concurrent gfs2_inode_lookup() and
gfs2_create_inode() calls, and the inode and iopen glocks may already be
pointing at another inode, so the checks in glock_clear_object() are
incorrect.

To better handle this case, always completely disassociate an inode from
its glocks before tearing it down.  In addition, get rid of a duplicate
glock_clear_object() call in gfs2_evict_inode().  That way,
glock_clear_object() will only ever be called when the glock points at
the current inode, and the NULL check in glock_clear_object() can be
removed.

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