Commit 17cf28af ("mm/fs: remove truncate_range") removed the
truncate_range inode operation in favour of the fallocate file
operation.

When using SYSV IPC shared memory segments, calling madvise with the
MADV_REMOVE advice on an area of shared memory will attempt to invoke
the .fallocate function for the shm_file_operations, which is NULL and
therefore returns -EOPNOTSUPP to userspace.  The previous behaviour
would inherit the inode_operations from the underlying tmpfs file and
invoke truncate_range there.

This patch restores the previous behaviour by wrapping the underlying
fallocate function in shm_fallocate, as we do for fsync.

[hughd@google.com: use -ENOTSUPP in shm_fallocate()]
Signed-off-by: Will Deacon <will.deacon@arm.com>
Acked-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>