Remediation for XFS/DIO corruption problem.

This adds a new volume option, shd-validate-data. When set, the self-heal code
will fetch checksums for regular files along with all the usual xattrs. If the
file seems OK but the checksums show a data mismatch, and if there is only one
replica that's out of step with the others, then we modify the source/sink
calculations to force a heal from one of the agreeing replicas to the odd one
out. Combined with a tool to put files into the self-heal index (being developed
separately), this provides a very rudimentary kind of scrubbing functionality.

Validation is now conditional on the "trusted.glusterfs.validate-status" xattr
having the specific value of "suspect" to avoid redoing validation (which is
expensive) as we find the same file in multiple bricks' indices. When we decide
to take action, we update this xattr to "clean" for copies that were in the
majority and "repaired" for the odd one out that gets clobbered. We also copy
the about-to-be-clobbered copy into an "orphans" directory to facilitate
analysis of corruption patterns. The data goes into ${GFID}.data there, while
${GFID}.link is a symlink to the file's old location.

Porting note: this is several internal squashed together ("See Also")

Differential Revision: https://phabricator.intern.facebook.com/D5092983
See Also: https://phabricator.intern.facebook.com/D5126974
See Also: https://phabricator.intern.facebook.com/D5127427
See Also: https://phabricator.intern.facebook.com/D5132804
See Also: https://phabricator.intern.facebook.com/D5209185
See Also: https://phabricator.intern.facebook.com/D5370353

Change-Id: Ie0ae18b368c408a5e47d0bf03ebac80b87b70aa9
Signed-off-by: Jeff Darcy <jdarcy@fb.com>
Reviewed-on: https://review.gluster.org/18269
Reviewed-by: Jeff Darcy <jeff@pl.atyp.us>
Tested-by: Jeff Darcy <jeff@pl.atyp.us>
CentOS-regression: Gluster Build System <jenkins@build.gluster.org>
Smoke: Gluster Build System <jenkins@build.gluster.org>

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