btrfs-progs: include headers required for musl-libc

This fixes various compilation errors where PATH_MAX and XATTR_SIZE_MAX
were missing. To my knowledge, this should have no bad side effects.

Signed-off-by: Merlijn Wajer <merlijn@wizzup.org>
Signed-off-by: David Sterba <dsterba@suse.cz>
btrfs-progs: disable backtrace and define __always_inline

Disable backtrace and define __always_inline when glibc is not used as
libc. This, together with some header changes allows btrfs-progs to
compile with musl-libc.

Signed-off-by: Merlijn Wajer <merlijn@wizzup.org>
Signed-off-by: David Sterba <dsterba@suse.cz>