file-posix, gluster: Handle zero block status hint better

file-posix, gluster: Handle zero block status hint better

Although the previous patch to change 'bool want_zero' into a bitmask
made no semantic change, it is now time to differentiate.  When the
caller specifically wants to know what parts of the file read as zero,
we need to use lseek and actually reporting holes, rather than
short-circuiting and advertising full allocation.

This change will be utilized in later patches to let mirroring
optimize for the case when the destination already reads as zeroes.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20250509204341.3553601-17-eblake@redhat.com>