fix IMA deadlock

[Christoph Hellwig]

IMA currently tries to read the file to calculate a hash from inside
i_rwsem.  On file system like XFS that also take i_rwsem for reads this
deadlocks.

This patch instead adds a new ->integrity_read method for IMA that can
be used to read data with i_rwsem held.  For XFS it has an implementation
that skips the locking, other file systems can just set it to the default
implementation.

As a side effect this also gets rid of playing with the address limit.
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