Partially revert r239959, after actually fixing most of the clang

Partially revert r239959, after actually fixing most of the clang
warnings in sys/gnu/fs/xfs.  The only warnings that still need to be
suppressed are those about array bound overruns of flexible array
members in xfs_dir2_{block,sf}.c, which are too expensive (in terms of
cascading code changes) to fix.

MFC after:	1 week
X-MFC-With:	r239959