innodb_log_file_mmap: Use a constant documentation string that
refers to persistent memory also when it is not available in the build.
HAVE_INNODB_MMAP: Remove, and unconditionally enable this code.
log_mmap(): On 32-bit systems, ensure that the size fits in 32 bits.
log_t::resize_start(), log_t::resize_abort(): Only handle memory-mapping
if HAVE_PMEM is defined. The generic memory-mapped interface is only for
reading the log in recovery. Writable memory mappings are only for
persistent memory, that is, Linux file systems with mount -o dax.
Reviewed by: Debarun Banerjee, Otto Kekäläinen
