EX-8270 ptlrpc: rename get_buf to get_pages

EX-8270 ptlrpc: rename get_buf to get_pages

The sptlrpc_enc_pool_get_buf function actually gets a fixed
number of pages, which is sort of a buffer, but is better
understood as a set of pages.

Rename the function for getting pages for a ptlrpc desc so
we can give get_buf a more appropriate name.

Test-Parameters: trivial
Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I9c03b9d638e7df7f09bf5724c5a6896b7d1e7b6c
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/52428
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>