[$] Improving fget() performance

The performance of the fget() function in the kernel was the topic
of a discussion led by Dave Watson at the 2019 Linux Storage, Filesystem,
and Memory-Management Summit (LSFMM).
fget()
is used to take a reference to a
file (i.e. bump a reference count), based on its file descriptor, and to
return the struct file pointer for it; references are dropped with
fput().
Some recent profiling at Watson's
employer, Facebook, found the function to be taking a sizable portion of
the CPU time for some applications, so he wanted to talk about some of the
things he has tried to make that situation better.