btrfs: allow to set compression level for zlib

Preliminary support for setting compression level for zlib, the
following works:

$ mount -o compess=zlib                 # default
$ mount -o compess=zlib0                # same
$ mount -o compess=zlib9                # level 9, slower sync, less data
$ mount -o compess=zlib1                # level 1, faster sync, more data
$ mount -o remount,compress=zlib3	# level set by remount

The compress-force works the same as compress'.  The level is visible in
the same format in /proc/mounts. Level set via file property does not
work yet.

Required patch: "btrfs: prepare for extensions in compression options"

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