GFS2: Lock i_mutex and use a local gfs2_holder for fallocate

This patch causes GFS2 to lock the i_mutex during fallocate. It
also switches from using a dinode's inode glock to using a local
holder like the other GFS2 i_operations.

Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>