gfs2: Eliminate ip->i_gh

Now that gfs2_file_buffered_write is the only remaining user of
ip->i_gh, we can move the glock holder to the stack (or rather, use the
one we already have on the stack); there is no need for keeping the
holder in the inode anymore.

This is slightly complicated by the fact that we're using ip->i_gh for
the statfs inode in gfs2_file_buffered_write as well.  Writing to the
statfs inode isn't very common, so allocate the statfs holder
dynamically when needed.

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