LU-14291 lustre: rename tgt_pool_* functions.

LU-14291 lustre: rename tgt_pool_* functions.

Functions starting with tgt_* represents code for target handling
used by Lustre servers. Now that the pool functions are used by
both clients and servers rename it to lu_tgt_* to mirror how
lu_tgt_desc_* is used since both represents remote server targets.

Test-Parameters: trivial
Change-Id: I2a9084d5bf9cea3b373c96e15cba1a41631d1172
Signed-off-by: James Simmons <jsimmons@infradead.org>
Reviewed-on: https://review.whamcloud.com/43624
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Aurelien Degremont <degremoa@amazon.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>