proc/kcore: don't walk list on every read

proc/kcore: don't walk list on every read

We maintain a list of memory ranges for /proc/kcore, which usually has
10-20 entries. Currently, every single read from /proc/kcore walks the
entire list in order to count the number of entries and compute some
offsets. These values only change when the list of memory ranges
changes, which is very rare (only when memory is hot(un)plugged). We can
cache the values when the list is populated to avoid these redundant
walks.

In my benchmark, this reduces the time per read by another 20
nanoseconds on top of the previous change, from 215 nanoseconds per read
to 195.

Link: https://github.com/osandov/drgn/issues/106
Signed-off-by: Omar Sandoval <osandov@fb.com>
Link: https://lore.kernel.org/r/8d945558b9c9efe74103a34b7780f1cd90d9ce7f.1731115587.git.osandov@fb.com
Signed-off-by: Christian Brauner <brauner@kernel.org>