btrfs: send: lower memory requirements in common case

The fs_path structure uses an inline buffer and falls back to a chain of
allocations, but vmalloc is not necessary because PATH_MAX fits into
PAGE_SIZE.

The size of fs_path has been reduced to 256 bytes from PAGE_SIZE,
usually 4k. Experimental measurements show that most paths on a single
filesystem do not exceed 200 bytes, and these get stored into the inline
buffer directly, which is now 230 bytes. Longer paths are kmalloced when
needed.

Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Josef Bacik <jbacik@fb.com>