xfs: scrub-related fixes

I discovered by sampling xfs_scrub stack trace swith a flame graph that
the attr scrub code has a sizeable oversight -- the xattr scrub code
always allocates a zeroed 65K temporary buffer before locking the inode,
even if it then turns out that the inode does not have extended
attributes.

In addition to the pointless memory allocation, the scrub code itself is
careful to initialize whatever part of the memory buffer it's going to
use before reading the contents, which means that the memory clearing is
not only painful (it's 5%% of the sample traces!) but totally pointless.

The first patch does more whack-a-mole cleanup of places where corrupt
ondisk directory metadata causes ASSERTs instead of -EFSCORRUPTED
returns.

The rest of the series first cleans up the open-coded pointer
calculations where the buffer is concerned, and then restructures the
code so to allocate the smallest size buffer needed and only just before
it's actually needed.  The final patch disables buffer zeroing for
better performance.