xfs_scrub: split up the mustfix repairs and difficulty assessment functions

xfs_scrub: split up the mustfix repairs and difficulty assessment functions

Currently, action_list_find_mustfix does two things -- it figures out
which repairs must be tried during phase 2 to enable the inode scan in
phase 3; and it figures out if xfs_scrub should warn about secondary and
primary metadata corruption that might make repair difficult.

Split these into separate functions to make each more coherent.  A long
time from now we'll need this to enable warnings about difficult rt
repairs, but for now this is merely a code cleanup.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>