gfs2: Stop using glock holder auto-demotion for now

We're having unresolved issues with the glock holder auto-demotion mechanism
introduced in commit dc732906.  This mechanism was assumed to be essential
for avoiding frequent short reads and writes until commit 296abc0d
("gfs2: No short reads or writes upon glock contention").  Since then,
when the inode glock is lost, it is simply re-acquired and the operation
is resumed.  This means that apart from the performance penalty, we
might as well drop the inode glock before faulting in pages, and
re-acquire it afterwards.

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