Btrfs: fix fspath error deallocation

Make sure to deallocate fspath with vfree() in case of error in
init_ipath().

fspath is allocated with vmalloc() in init_data_container() since
commit 425d17a2

 ("Btrfs: use larger limit for translation of logical to
inode").

Signed-off-by: Vincent Stehlé <vincent.stehle@intel.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>