ocfs2: fix incorrect i_size of global bitmap inode after resize

Ocfs2 cluster size may be 1MB, which has 20 bits.  When resize, the
input new clusters is mostly the number of clusters in a group
descriptor(32256).

Since the input clusters is defined as type int, so it will overflow
when shift left 20 bits and then lead to incorrect global bitmap i_size.

Signed-off-by: Joseph Qi <joseph.qi@huawei.com>
Cc: Mark Fasheh <mfasheh@suse.com>
Cc: Joel Becker <jlbec@evilplan.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>