xfs_db: fix nbits parameter in fa_ino[48] functions

xfs_db: fix nbits parameter in fa_ino[48] functions

Use the proper macro to convert ino4 and ino8 field byte sizes to a bit
count in the functions that navigate shortform directories.  This just
happens to work correctly for ino4 entries, but omits the upper 4 bytes
of an ino8 entry.  Note that the entries display correctly; it's just
the command "addr u3.sfdir3.list[X].inumber.i8" that won't.

Found by running smatch.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>