[PATCH v2] Backport xfs security fix to 4.9 and 4.4 stable trees

[Siddharth Chandrasekaran <csiddharth@vmware.com>]

Hello,

Lack of proper validation that cached inodes are free during allocation can,
cause a crash in fs/xfs/xfs_icache.c (refer: CVE-2018-13093). To address this
issue, I'm backporting upstream commit [1] to 4.4 and 4.9 stable trees
(a backport of [1] to 4.14 already exists).

Also, commit [1] references another commit [2] which added checks only to
xfs_iget_cache_miss(). In this patch, those checks have been moved into a
dedicated checker method and both xfs_iget_cache_miss() and
xfs_iget_cache_hit() are made to call that method. This code reorg in commit
[1], makes commit [2] redundant in the history of the 4.9 and 4.4 stable
trees. So commit [2] is not being backported.

-- Sid

[1]: afca6c5b2595 ("xfs: validate cached inodes are free when allocated")
[2]: ee457001ed6c ("xfs: catch inode allocation state mismatch corruption")

change log:
v2:
 - Reword cover letter.
 - Fix accidental worong patch that got mailed.

-- 
2.7.4