LU-15487 mdd: print FID in mdd_dir_page_build() error

LU-15487 mdd: print FID in mdd_dir_page_build() error

Print the MDT name and FID in mdd_dir_page_build() when an error
is hit.  Because this changes the callback function signature,
also update dt_index_page_build() to print a more useful message.

Add OBD_FAIL_MDS_DIR_PAGE_WALK to allow triggering this codepath
to see if this is the source of problems in error handling.

Minor code style and whitespace cleanups in related functions.

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Ic475f4a2c775871ff5af59a47e0966ba3eed7013
Reviewed-on: https://review.whamcloud.com/46368
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>