ntfs: ->d_compare() must not block

ntfs: ->d_compare() must not block

... so don't use __getname() there.  Switch it (and ntfs_d_hash(), while
we are at it) to kmalloc(PATH_MAX, GFP_NOWAIT).  Yes, ntfs_d_hash()
almost certainly can do with smaller allocations, but let ntfs folks
deal with that - keep the allocation size as-is for now.

Stop abusing names_cachep in ntfs, period - various uses of that thing
in there have nothing to do with pathnames; just use k[mz]alloc() and
be done with that.  For now let's keep sizes as-in, but AFAICS none of
the users actually want PATH_MAX.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>