[PATCH] shmem: restore superblock info

To improve shmem scalability, we allowed tmpfs instances which don't need
their blocks or inodes limited not to count them, and not to allocate any
sbinfo.  Which was okay when the only use for the sbinfo was accounting
blocks and inodes; but since then a couple of unrelated projects extending
tmpfs want to store other data in the sbinfo.  Whether either extension
reaches mainline is beside the point: I'm guilty of a bad design decision,
and should restore sbinfo to make any such future extensions easier.

So, once again allocate a shmem_sb_info for every shmem/tmpfs instance, and
now let max_blocks 0 indicate unlimited blocks, and max_inodes 0 unlimited
inodes.  Brent Casavant verified (many months ago) that this does not
perceptibly impact the scalability (since the unlimited sbinfo cacheline is
repeatedly accessed but only once dirtied).

And merge shmem_set_size into its sole caller shmem_remount_fs.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

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