hfs: fix a potential buffer overflow

A specially-crafted Hierarchical File System (HFS) filesystem could cause
a buffer overflow to occur in a process's kernel stack during a memcpy()
call within the hfs_bnode_read() function (at fs/hfs/bnode.c:24).  The
attacker can provide the source buffer and length, and the destination
buffer is a local variable of a fixed length.  This local variable (passed
as "&entry" from fs/hfs/dir.c:112 and allocated on line 60) is stored in
the stack frame of hfs_bnode_read()'s caller, which is hfs_readdir().
Because the hfs_readdir() function executes upon any attempt to read a
directory on the filesystem, it gets called whenever a user attempts to
inspect any filesystem contents.

[amwang@redhat.com: modify this patch and fix coding style problems]
Signed-off-by: WANG Cong <amwang@redhat.com>
Cc: Eugene Teo <eteo@redhat.com>
Cc: Roman Zippel <zippel@linux-m68k.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Dave Anderson <anderson@redhat.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>