btrfs-progs: don't use <linux/fs.h>

btrfs-progs: don't use <linux/fs.h>

sparse can freak out when <linux/fs.h> is included because it redefines
approximately a gazillion symbols already found in <sys/mount.h>:

/usr/include/linux/fs.h:203:9: warning: preprocessor token MS_RDONLY redefined
/usr/include/sys/mount.h:37:9: this was the original definition

Happily, we don't actually need to include the low-level <linux/fs.h>
for anything.  One assumes it was just carried over from kernel space.

Signed-off-by: Zach Brown <zab@redhat.com>
Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>