rafs: fix bug in InodeWrapper::is_sock()

[SToPire]

We incorrectly use is_dir() to check if a file is a socket. This patch fixes it.

Types of changes

What types of changes does your PullRequest introduce? Put an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation Update (if none of the other choices apply)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 61.28%. Comparing base (9f439ab) to head (9dc005d).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1630      +/-   ##
==========================================
- Coverage   61.28%   61.28%   -0.01%     
==========================================
  Files         146      146              
  Lines       48156    48156              
  Branches    46123    46123              
==========================================
- Hits        29514    29510       -4     
- Misses      17084    17085       +1     
- Partials     1558     1561       +3     

Files with missing lines	Coverage Δ
rafs/src/metadata/inode.rs	95.39% <100.00%> (ø)

... and 3 files with indirect coverage changes

[imeoer]

LGTM, thanks!