fs/minix: check return value of sb_getblk()

fs/minix: check return value of sb_getblk()

Patch series "fs/minix: fix syzbot bugs and set s_maxbytes".

This series fixes all syzbot bugs in the minix filesystem:

	KASAN: null-ptr-deref Write in get_block
	KASAN: use-after-free Write in get_block
	KASAN: use-after-free Read in get_block
	WARNING in inc_nlink
	KMSAN: uninit-value in get_block
	WARNING in drop_nlink

It also fixes the minix filesystem to set s_maxbytes correctly, so that
userspace sees the correct behavior when exceeding the max file size.

This patch (of 6):

sb_getblk() can fail, so check its return value.

This fixes a NULL pointer dereference.

Originally from Qiujun Huang.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-by: syzbot+4a88b2b9dc280f47baf4@syzkaller.appspotmail.com
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Qiujun Huang <anenbupt@gmail.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: <stable@vger.kernel.org>
Link: http://lkml.kernel.org/r/20200628060846.682158-1-ebiggers@kernel.org
Link: http://lkml.kernel.org/r/20200628060846.682158-2-ebiggers@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>