LU-13163 mdc: new kernel function xa_is_value()

LU-13163 mdc: new kernel function xa_is_value()

xa_is_value() is added in kernel 4.19-rc6 to replace
radix_tree_entry_exceptional().

This patch is back-ported from the following one:
Lustre-commit: a0a3a29deb82656f9639f46847deac2689973893
Lustre-change: https://review.whamcloud.com/37399

Test-Parameters: trivial clientdistro=el8.1 \
envdefinitions=ONLY=65i mdscount=2 mdtcount=4 \
testlist=sanity,sanity,sanity,sanity,sanity

Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: If89aa19c37af8a67debe782d1c77f4ef4dc6f923
Reviewed-on: https://review.whamcloud.com/37481
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>