GFS2: Get a block reservation before resizing a file

commit d2b47cfb upstream.

This patch allocates a block reservation structure before growing
or shrinking a file. Without this structure, the grow or shink code
can reference the bad pointer.

Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>