LU-12137 osd-ldiskfs: migrate ll_lookup_one_len to osd_compat.c

LU-12137 osd-ldiskfs: migrate ll_lookup_one_len to osd_compat.c

The function ll_lookup_one_len() in lvfs.h is only used for the
osd-ldiskfs layer so relocate it to osd_compat.c and rename it
to osd_lookup_one_len_unlocked(). We use unlocked in the name
since this signifies it must always be called with the inode lock
not taken since it will take the inode lock itself.

Test-Parameters: trivial

Change-Id: I507100a676f98448f4cacc94e902294903a67efb
Signed-off-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-on: https://review.whamcloud.com/35454
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Neil Brown <neilb@suse.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>