linux: Fix ENOTSOCK handling in sendfile(2)

The Linux way for sendfile(2) to tell the application
to fallback to another way of copying data is by EINVAL,
not ENOTSOCK. This fixes package installation scripts
for Mono packages from Focal.

Sponsored By: EPSRC
Differential Revision: https://reviews.freebsd.org/D32604

(cherry picked from commit 2c7f7982825442c851e8bb17555a086a232ef4d9)