Fix up MaxRead/MaxWrite handling

Linux clients at least restrict MaxRead/MaxWrite to no larger than what
is offered on the Pseudo FS export. Thus we default that to 4 MB.

Also, define an FSAL_MAXIO constant to use for this.

Also, if an FSAL provides 0, properly treat that as no maximum.

Signed-off-by: Frank S. Filz <ffilzlnx@mindspring.com>

Conflicts:
	src/FSAL/FSAL_PSEUDO/main.c
Use UINT64_MAX for maxfilesize instead of hard coded constant

Signed-off-by: Frank S. Filz <ffilzlnx@mindspring.com>