xfs: enable use of log incompat features

Currently, there are no bits defined for the log incompat feature field
of V5 superblocks.  This field is intended to be used for enabling new
log features that older kernels cannot recover, such as new log intent
items, when those new features do not affect the ondisk format.

The upcoming atomic extent swap and logged xattrs features require new
log intent items to track high level file operations.  Therefore, they
need a way to turn on the feature in the superblock to prevent replay on
old kernels, and they need a way to turn off the sb bit once the new log
becomes clean again.  This patchset is short -- the first patch enables
high level code to turn on a log-incompat feature bit, and the second
patch turns it off again when we write a clean log to disk.
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