shmem: update memory reservation on truncate

A shared anonymous mapping created without MAP_NORESERVE holds memory
reservation for whole range of shmem segment.  Usually there is no way
to change its size, but /proc/<pid>/map_files/...  (available if
CONFIG_CHECKPOINT_RESTORE=y) allows that.

This patch adjusts the memory reservation in shmem_setattr().

Signed-off-by: Konstantin Khlebnikov <koct9i@gmail.com>
Acked-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>