LU-14195 osd: don't use set_fs() for ->fiemap() calls.

LU-14195 osd: don't use set_fs() for ->fiemap() calls.

->fiemap() only accesses kernel-space data, so does not need, and
never has needed, set_fs() calls.
In Linux 5.10, these calls are deprecated.
So remove the unnecessary code.

Lustre-change: https://review.whamcloud.com/40909
Lustre-commit: d0337cab8e845efcdbfb9e26e573feb18f28e303

Test-Parameters: trivial
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: Id336855b4787ddbf656dfa3b8d0b12f663564795
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/44142
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Li Xi <lixi@ddn.com>