LinuxKPI: add fsleep()

Add fsleep() function now required by rtw88. This seems to be
making a decision depending on time to sleep on how to sleep.
Given our compat framework already is lenient on how long to sleep,
this is a cut down version.

MFC after: 10 days
Reviewed by: hselasky
Differential Revision: https://reviews.freebsd.org/D31322