btrfs: send: make use of -fms-extensions for defining struct fs_path

btrfs: send: make use of -fms-extensions for defining struct fs_path

The newly introduced -fms-extensions compiler flag allows defining
struct fs_path in such a way that inline_buf becomes a proper array
with a size known to the compiler.

This also makes the problem fixed by commit 8aec9dbf2db2 ("btrfs: send:
fix -Wflex-array-member-not-at-end warning in struct send_ctx") go away.
Whether cur_inode_path should be put back to its original place in
struct send_ctx I don't know, but at least the comment no longer
applies.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Acked-by: David Sterba <dsterba@suse.com>
Link: https://patch.msgid.link/20251020142228.1819871-3-linux@rasmusvillemoes.dk
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Nicolas Schier <nsc@kernel.org>