cp: use FICLONE instead of BTRFS_IOC_CLONE

cp: use FICLONE instead of BTRFS_IOC_CLONE

This doesn't affect the generated code on my system; it's merely
a cleaner way to use the recently-introduced Linux-specific API.
* m4/jm-macros.m4 (gl_CHECK_ALL_HEADERS): Check for linux/fs.h.
* src/copy.c: Include <linux.fs.h> if available.
(FICLONE) [__linux__]: Define if not already defined.
(clone_file): Use FICLONE instead of the older BTRFS_IOC_CLONE,
as this ioctl is no longer btrfs-specific.