xfs: clean up scrub context if scrub setup returns -EDEADLOCK

It has been a longstanding convention that online scrub and repair
functions can return -EDEADLOCK to signal that they weren't able to
obtain some necessary resource.  When this happens, the scrub framework
is supposed to release all resources attached to the scrub context, set
the TRY_HARDER flag in the scrub context flags, and try again.  In this
context, individual scrub functions are supposed to take all the
resources they (incorrectly) speculated were not necessary.

We're about to make it so that the functions that lock and wait for a
filesystem AG can also return EDEADLOCK to signal that we need to try
again with the drain waiters enabled.  Therefore, refactor
xfs_scrub_metadata to support this behavior for ->setup() functions.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>