reiserfs: fix broken xattr handling (heap corruption, bad retval)

reiserfs: fix broken xattr handling (heap corruption, bad retval)

This fixes the following issues:

- When a buffer size is supplied to reiserfs_listxattr() such that each
  individual name fits, but the concatenation of all names doesn't fit,
  reiserfs_listxattr() overflows the supplied buffer.  This leads to a
  kernel heap overflow (verified using KASAN) followed by an out-of-bounds
  usercopy and is therefore a security bug.

- When a buffer size is supplied to reiserfs_listxattr() such that a
  name doesn't fit, -ERANGE should be returned.  But reiserfs instead just
  truncates the list of names; I have verified that if the only xattr on a
  file has a longer name than the supplied buffer length, listxattr()
  incorrectly returns zero.

With my patch applied, -ERANGE is returned in both cases and the memory
corruption doesn't happen anymore.

Credit for making me clean this code up a bit goes to Al Viro, who pointed
out that the ->actor calling convention is suboptimal and should be
changed.

Link: http://lkml.kernel.org/r/20180802151539.5373-1-jannh@google.com
Fixes: 48b32a3553a5 ("reiserfs: use generic xattr handlers")
Signed-off-by: Jann Horn <jannh@google.com>
Acked-by: Jeff Mahoney <jeffm@suse.com>
Cc: Eric Biggers <ebiggers@google.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>