Some might want a tmpfs mount with the improved scalability afforded by
omitting shmem superblock accounting; or some might just want to test it in an
externally-visible tmpfs mount instance.

Adopt the convention that mount option -o nr_blocks=0,nr_inodes=0 means
without resource limits, and hence no shmem_sb_info.  Not recommended for
general use, but no worse than ramfs.

Disallow remounting from unlimited to limited (no accounting has been done so
far, so no idea whether it's permissible), and from limited to unlimited
(because we'd need then to free the sbinfo, and visit each inode to reset its
i_blocks to 0: why bother?).
Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>