xfs: random SWAPEXT fixes

This series fixes some problems I found in the extent swap code.  The
first patch makes sure we drop the inode lock when cleaning out the COW
fork fails.  The second patch turns some metadata ASSERTs into a proper
EFSCORRUPTED return.  The third patch teaches swapext to adjust the
quota counts properly, which fixes a bug where unprivileged userspace
can cause the quota counts to be wrong by calling swapext on a pair of
files with different uid/gid/prid.
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