[GFS2] Fix page lock/glock deadlock

This fixes a race between the glock and the page lock encountered
during truncate in gfs2_readpage and gfs2_prepare_write. The gfs2_readpages
function doesn't need the same fix since it only uses a try lock anyway, so
it will fail back to gfs2_readpage in the case of a potential deadlock.

This bug was spotted by Russell Cattelan.

Cc: Russell Cattelan <cattelan@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>