tmpfs: enforce size limit on writes when file system size is default

tmpfs enforced the file system size limit on writes for file systems
with a specified size, but not when the size was the default. Add
enforcement when the size is default: do not allocate additional
pages if the available memory + swap falls to the reserve level.
Note, enforcement is also done when attempting to create a file,
both with and without an explicit file system size.

PR: 275436
MFC after: 1 month
Reviewed by: cy
Differential Revision: https://reviews.freebsd.org/D43010