block/export/fuse: Fix build failure on FreeBSD

block/export/fuse: Fix build failure on FreeBSD

When building on FreeBSD we get:

  [816/6851] Compiling C object libblockdev.fa.p/block_export_fuse.c.o
  ../block/export/fuse.c:628:16: error: use of undeclared identifier 'FALLOC_FL_KEEP_SIZE'
      if (mode & FALLOC_FL_KEEP_SIZE) {
                 ^
  ../block/export/fuse.c:651:16: error: use of undeclared identifier 'FALLOC_FL_PUNCH_HOLE'
      if (mode & FALLOC_FL_PUNCH_HOLE) {
                 ^
  ../block/export/fuse.c:652:22: error: use of undeclared identifier 'FALLOC_FL_KEEP_SIZE'
          if (!(mode & FALLOC_FL_KEEP_SIZE)) {
                       ^
  3 errors generated.
  FAILED: libblockdev.fa.p/block_export_fuse.c.o

Meson indeed reported FALLOC_FL_PUNCH_HOLE is not available:

  C compiler for the host machine: cc (clang 10.0.1 "FreeBSD clang version 10.0.1")
  Checking for function "fallocate" : NO
  Checking for function "posix_fallocate" : YES
  Header <linux/falloc.h> has symbol "FALLOC_FL_PUNCH_HOLE" : NO
  Header <linux/falloc.h> has symbol "FALLOC_FL_ZERO_RANGE" : NO
  ...

Similarly to commit 304332039 ("block/export/fuse.c: fix musl build"),
guard the code requiring FALLOC_FL_KEEP_SIZE / FALLOC_FL_PUNCH_HOLE
definitions under CONFIG_FALLOCATE_PUNCH_HOLE #ifdef'ry.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20220201112655.344373-3-f4bug@amsat.org>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>