Candidate fix for PR kern/50607, PR port-evbmips/50059.
Formerly VOP_WRITE-->ext2fs_write would automatically dispatch to
this code path for writes to directories, but I broke that in
ext2fs_lookup.c rev. 1.78 when disentangling page-cached and
buffer-cached writes.
This was not a problem in ufs, and I didn't notice it in ext2fs,
because ufs consistently used buffercache(9) directly instead of
using VOP_WRITE sometimes as ext2fs did.