nilfs2: fix potential use after free in nilfs_gccache_submit_read_data()

In the Linux kernel, the following vulnerability has been resolved:

nilfs2: fix potential use after free in nilfs_gccache_submit_read_data()

In nilfs_gccache_submit_read_data(), brelse(bh) is called to drop the

reference count of bh when the call to nilfs_dat_translate() fails. If

the reference count hits 0 and its owner page gets unlocked, bh may be

freed. However, bh->b_page is dereferenced to put the page after that,

which may result in a use-after-free bug. This patch moves the release

operation after unlocking and putting the page.

NOTE: The function in question is only called in GC, and in combination

with current userland tools, address translation using DAT does not occur

in that function, so the code path that causes this issue will not be

executed. However, it is possible to run that code path by intentionally

modifying the userland GC library or by calling the GC ioctl directly.

[[email protected]: NOTE added to the commit log]