btrfs-progs: build, fix rules to clean temporary files in docs

btrfs-progs: build, fix rules to clean temporary files in docs

$ make clean-doc
Cleaning Documentation
/usr/bin/rm: cannot remove ‘*.xml’: No such file or directory
/usr/bin/rm: cannot remove ‘*.xml+’: No such file or directory
/usr/bin/rm: cannot remove ‘*.5’: No such file or directory
/usr/bin/rm: cannot remove ‘*.8’: No such file or directory
make[1]: *** [clean] Error 1
make: *** [clean-doc] Error 2

The RM variable from parent makefile lacks -f, add it where it's
missing.
Minor change in LN_S variable name, same -f change.

Signed-off-by: David Sterba <dsterba@suse.cz>