Btrfs: check return value of btrfs_alloc_path()

Adding the check on the return value of btrfs_alloc_path() to several places.
And, some of callers are modified by this change.

Signed-off-by: Tsutomu Itoh <t-itoh@jp.fujitsu.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>