mmap: fd should be -1 when creating anonymous mappings

[svenski123]

Allocating anonymous with mmap() should have fd set to -1 in conforming code (as 0 is a valid value for a file descriptor).
Linux is forgiving about this however other OS's (FreeBSD) less so.
This trivial change fixes BPF programs on Solana/FreeBSD which are currently broken by this.
I have copied an extract from the linux 2 mmap page for reference below.

MAP_ANONYMOUS
The mapping is not backed by any file; its contents are initialized to zero.  The fd argument is ignored; however, some implementations
require fd to be -1 if MAP_ANONYMOUS (or MAP_ANON) is specified, and portable applications should ensure  this.   The  offset  argument
should be zero.  The use of MAP_ANONYMOUS in conjunction with MAP_SHARED is supported on Linux only since kernel 2.4.

[codecov-commenter]

Codecov Report

Merging #508 (5fe3c71) into main (1bd6034) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #508      +/-   ##
==========================================
- Coverage   89.62%   89.62%   -0.01%     
==========================================
  Files          23       23              
  Lines       10297    10294       -3     
==========================================
- Hits         9229     9226       -3     
  Misses       1068     1068              

Files Changed	Coverage Δ
src/memory_management.rs	94.00% <ø> (ø)

... and 2 files with indirect coverage changes