tmpfs: increase memory reserve to a percent of available memory + swap

The tmpfs memory reserve defaulted to 4 MB, and other than that,
all of available memory + swap could be allocated to tmpfs files.
This was dangerous, as the page daemon attempts to keep some memory
free, using up swap, and then resulting in processes being killed.
Increase the reserve to a fraction of available memory + swap at
file system startup time. The limit is expressed as a percentage
of available memory + swap that can be used, and defaults to 95%.
The percentage can be changed via the vfs.tmpfs.memory_percent sysctl,
recomputing the reserve with the new percentage but the initial
available memory + swap. Note that the reserve can also be set
directly with an existing sysctl, ignoring the percentage. The
previous behavior can be specified by setting vfs.tmpfs.memory_percent
to 100.

Add sysctl for vfs.tmpfs.memory_percent and the pre-existing
vfs.tmpfs.memory_reserved to tmpfs(5).

PR: 275436
MFC after: 1 month
Reviewed by: rgrimes
Differential Revision: https://reviews.freebsd.org/D43011