LU-18653 utils: add "--clear-fixed" to "lfs migrate" help

LU-18653 utils: add "--clear-fixed" to "lfs migrate" help

Add the "--clear-fixed" option to the "lfs migrate" help string.
And correct the lfs-migrate.1 manpage:
s/LMV_HASH_FIXED_FLAG/LMV_HASH_FLAG_FIXED/

Test-Parameters: trivial
Fixes: 392f558f40 ("LU-17810 dne: dir restripe without fixed hash flag")
Signed-off-by: Etienne AUJAMES <etienne.aujames@cea.fr>
Change-Id: Ie5db5264777b036043f73453f22bebdf58f8ebcd
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/57814
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Timothy Day <timday@amazon.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>