xfs_repair: improve checking of existing rmap and refcount btrees

xfs_repair: improve checking of existing rmap and refcount btrees

There are a few deficiencies in the xfs_repair functions that check the
existing reverse mapping and reference count btrees.  First of all, we
don't report corruption or IO errors if we can't read the ondisk
metadata.  Second, we don't consistently warn if we cannot allocate
memory to perform the check.

Add the missing warnings, and simplify the calling convention since we
don't need the extra layer of do_error logging in phase4.c.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>